Introduction
Crohn’s vs Ulcerative Colitis: Key Differences
Last Updated Dec 3, 2025
Crohn’s disease and ulcerative colitis are the two main types of inflammatory bowel disease. They share many symptoms, yet where and how they cause inflammation is different. Understanding these patterns helps patients, families, and clinicians choose tests, medicines, and surgery, and understand why complications and long term plans are not the same.
Key Takeaways
Crohn’s disease can affect any part of the digestive tract, while ulcerative colitis is limited to the colon and rectum.
Crohn’s inflammation often affects the full thickness of the bowel wall; ulcerative colitis mainly involves the inner lining. (mayoclinic.org)
Crohn’s is more likely to cause strictures, fistulas, and perianal disease, while ulcerative colitis is more linked with severe colitis and toxic megacolon. (pubmed.ncbi.nlm.nih.gov)
Colonoscopy and biopsies usually distinguish the two, but about 5 to 15 percent of people have “IBD-unclassified” when features overlap. (pubmed.ncbi.nlm.nih.gov)
Surgery can cure colonic ulcerative colitis by removing the colon and rectum; surgery rarely cures Crohn’s disease because inflammation can return elsewhere. (mayoclinic.org)
Big picture: how Crohn’s and ulcerative colitis compare
Both Crohn’s disease and ulcerative colitis are chronic forms of inflammatory bowel disease (IBD). They flare and quiet down over time, and both can cause diarrhea, pain, fatigue, and weight loss.
However, they differ in where inflammation appears, how deep it goes, and which complications tend to develop. These differences guide tests, medication choices, and surgical planning.
Snapshot comparison
Feature | Crohn’s disease | Ulcerative colitis |
|---|---|---|
Main locations | Anywhere from mouth to anus, most often end of small intestine and right colon | Only colon and rectum |
Pattern | Patchy “skip” areas with normal tissue between inflamed spots | Continuous inflammation starting in rectum and spreading up the colon |
Depth of inflammation | Often full thickness of bowel wall (transmural) | Mainly inner lining (mucosa) |
Typical complications | Strictures, fistulas, abscesses, perianal disease | Acute severe colitis, toxic megacolon, higher colon cancer risk with long-standing pancolitis |
Effect of removing colon | Symptoms can improve but disease can return elsewhere | Removing colon and rectum can cure colonic disease |
Where inflammation occurs: distribution
Crohn’s: anywhere in the digestive tract, patchy pattern
In Crohn’s disease, inflammation can appear anywhere in the gastrointestinal tract, from the mouth to the anus. It most often affects the last part of the small intestine (terminal ileum) and parts of the colon. (mayoclinic.org)
The pattern is usually patchy. Areas of diseased tissue are separated by segments of normal-looking bowel, called skip lesions. The rectum may be normal, mildly inflamed, or severely involved. Perianal problems such as fissures, abscesses, and fistulas are much more common in Crohn’s than in ulcerative colitis. (pubmed.ncbi.nlm.nih.gov)
Ulcerative colitis: limited to colon and rectum, continuous pattern
In ulcerative colitis, inflammation is limited to the colon and rectum. It almost always starts in the rectum and then extends upward in a continuous fashion without normal “skip” areas. (mayoclinic.org)
Disease may involve only the rectum (proctitis), the left side of the colon, or the entire colon (pancolitis). Classic teaching is that only the colon is affected, though rare backwash inflammation in the very end of the ileum can occur when the colon is severely involved.
How deep inflammation goes
Crohn’s: full-thickness (transmural) inflammation
Crohn’s inflammation often extends through the entire bowel wall, from inner lining to outer surface. This transmural pattern explains several common Crohn’s complications:
Strictures from scarring and narrowing of the bowel
Fistulas (tunnels) between bowel loops or from bowel to skin or nearby organs
Abscesses (pockets of infection) around the bowel or anus
Ulcerative colitis: mainly mucosal inflammation
Ulcerative colitis mainly affects the mucosa, the innermost lining of the colon, and sometimes the immediate layer underneath (submucosa). This is why bleeding and mucus in the stool are common, while deep fistulas and strictures are less common than in Crohn’s. (mayoclinic.org)
Newer imaging and pathology studies suggest that deeper wall changes can occur in some cases of long-standing or severe ulcerative colitis, but clinically it is still considered primarily a mucosal disease. (pubmed.ncbi.nlm.nih.gov)
Complications: what tends to differ
Complications seen more often in Crohn’s
Because Crohn’s can involve any segment and the full bowel wall, certain problems are more typical:
Intestinal strictures that cause cramping, bloating, and bowel obstruction
Fistulas and abscesses, especially around the anus or between bowel loops
Perianal disease with painful fissures, drainage, and skin tags
Malabsorption and growth delay when the small intestine is affected, especially in children
Recurrent disease after surgery, often at the edge of a surgical join
Complications seen more often in ulcerative colitis
In ulcerative colitis, complications relate to widespread colon inflammation:
Acute severe colitis with very frequent bloody stools and high inflammation
Toxic megacolon, when the colon becomes dangerously dilated and at risk of perforation
Massive bleeding or perforation in rare, severe cases
Increased colon cancer risk with long-standing, extensive colitis, especially with associated primary sclerosing cholangitis (PSC)
Complications shared by both
Both Crohn’s and ulcerative colitis can cause:
Anemia, weight loss, and fatigue from chronic inflammation or blood loss
Bone loss and nutritional deficiencies
Colorectal cancer risk when a large part of the colon has been inflamed for many years
Extraintestinal manifestations, such as joint pain, certain eye inflammations, skin lesions, and liver conditions
Cancer surveillance strategies and timing are similar when the colon is extensively involved in either condition. (mayoclinic.org)
How doctors tell Crohn’s and ulcerative colitis apart
No single “gold standard” test
There is no single test that proves Crohn’s or ulcerative colitis on its own. Diagnosis relies on a combination of:
Symptom history and physical examination
Endoscopy (colonoscopy or flexible sigmoidoscopy)
Biopsies examined under a microscope
Imaging of the small bowel and colon
Blood and stool tests
Clinicians put these findings together to classify disease as Crohn’s, ulcerative colitis, or IBD-unclassified. (mdpi.com)
Endoscopy findings
On colonoscopy:
Crohn’s disease often shows patchy inflammation, deep linear ulcers, a cobblestone appearance, and areas of normal mucosa between diseased segments. The rectum can be spared.
Ulcerative colitis shows continuous, symmetric inflammation starting in the rectum and extending proximally, with granular, friable mucosa and superficial ulceration.
Biopsy (pathology) findings
Under the microscope:
Noncaseating granulomas that are away from areas of crypt damage are highly specific for Crohn’s disease, although they are not present in every sample.
In ulcerative colitis, inflammation is diffuse in the mucosa, with crypt architectural distortion and crypt abscesses, but without granulomas.
Pathologists also look at depth of inflammation, distribution, and any features that do not fit IBD, such as infection or ischemia.
Imaging
Cross-sectional imaging, such as magnetic resonance enterography (MRE), CT enterography, or intestinal ultrasound, helps identify:
Small bowel involvement that favors Crohn’s disease
Bowel wall thickening patterns
Fistulas, abscesses, or strictures
Capsule endoscopy can detect subtle small bowel Crohn’s when colonoscopy appears normal. (mdpi.com)
Blood and stool markers
Blood and stool tests show inflammation but rarely define the exact type of IBD:
C-reactive protein (CRP) and fecal calprotectin rise with active bowel inflammation in both conditions.
Certain antibodies, such as pANCA (more common in ulcerative colitis) and ASCA (more common in Crohn’s), can support but not replace other diagnostic information. (pubmed.ncbi.nlm.nih.gov)
When the diagnosis is not clear: IBD-unclassified
In some patients, colonoscopy, biopsies, and imaging do not clearly match classic Crohn’s or classic ulcerative colitis. In these cases, the label IBD-unclassified (IBD-U) or indeterminate colitis may be used.
IBD-U describes about 5 to 15 percent of people with IBD. Over time, many of these patients eventually meet clear criteria for Crohn’s disease or ulcerative colitis as new information appears or the disease pattern evolves. (pubmed.ncbi.nlm.nih.gov)
Treatment in IBD-U often follows the approach that best fits the current pattern of inflammation and severity while keeping surgical decisions, such as creating a pouch, under careful review.
FAQs
Can Crohn’s turn into ulcerative colitis, or the other way around?
The underlying disease type does not truly “change,” but the label can change as more information is gathered. A small percentage of patients first diagnosed with ulcerative colitis are later reclassified as Crohn’s disease, and a smaller fraction of Crohn’s cases are reclassified as ulcerative colitis. (mdpi.com)
Is surgery a cure for both Crohn’s and ulcerative colitis?
Removing the colon and rectum can cure colonic ulcerative colitis, since the disease is limited to those organs. In Crohn’s disease, surgery can remove damaged segments and treat complications, but inflammation can return in other parts of the digestive tract, so it is not considered a cure. (mayoclinic.org)
Why do some people receive an “IBD-unclassified” diagnosis?
IBD-unclassified is used when features of Crohn’s and ulcerative colitis overlap and a definite classification is not possible. This situation is more common in children and in people with very severe or early disease. Many individuals with IBD-U later develop clearer features that allow doctors to reclassify the condition as Crohn’s disease or ulcerative colitis. (pubmed.ncbi.nlm.nih.gov)