In a prospective study, 118 patients with Crohn's disease, 51 patients with ulcerative colitis, and 72 patients with no disease of the intestine proximal to the rectum were evaluated by ultrasound. In Crohn's disease, thickening of the bowel wall and inflammatory masses were detected in 72.08 of the patients. With a transducer having optimal imaging properties in the near range, these findings were detected in 87.2% of a group of 47 patients. In ulcerative colitis, bowel wall thickening was detected in 52.9% of all patients. Thickening of the bowel wall was more marked in Crohn's disease than in ulcerative colitis. Most pathologic findings in Crohn's disease were located in the right lower abdomen, whereas those in ulcerative colitis were in the left abdomen, in particular in the lower quadrant. The frequency of wall thickening was correlated to the activity of the disease in ulcerative colitis but not in Crohn's disease. Considerably increased wall thickness, when localized in the right lower quadrant and found in combination with inflammatory masses or an abscess, suggests Crohn's disease.