The distinction between ulcerative colitis and Crohn's colitis has become crucially important since the introduction of pouch surgery. The classic pathologic findings in ulcerative colitis and Crohn's colitis are described, as well as the subtleties causing the difficulty in differentiation. In difficult cases, it is essential to review the whole patient, all available biopsy material, and the colectomy specimen. The diverted rectum in ulcerative colitis usually becomes more inflamed, whereas in Crohn's disease inflammation usually resolves. Upper gastrointestinal tract biopsies with apparent normal mucosa may also be contributory.