PURPOSE Colonic diverticulosis is characterized by abnormal thickening of the bowel wall, associated with luminal overpressure and increase of sigmoid contractility. However, patients with ulcerative colitis show chronic inflammatory alterations determining a reduction of both bowel wall muscle tone and contractility. Thus, we could presume ulcerative colitis and colonic diverticulosis as two pathophysiologically and mutually excluding diseases. This study was designed to evaluate the prevalence of colonic diverticulosis in patients with ulcerative colitis compared with a control endoscopic population. METHODS We prospectively analyzed the prevalence of colonic diverticulosis in 85 patients, older than aged 45 years, with known ulcerative colitis compared with that in 85 age/gender-matched patients without colitis. All patients underwent pancolonoscopy with ulcerative colitis and colonic diverticulosis diagnosis made by endoscopy and histopathology. The patients with ulcerative colitis also were divided in three subgroups according to the age at diagnosis (<30 years, 30-45 years, >45 years) and extension of disease (sigmoiditis, left colitis, extensive colitis). RESULTS Colonic diverticulosis was present in 7 of 85 patients with and in 24 patients without ulcerative colitis (8.2 vs. 28.2 percent; P < 0.001; relative risk, 3.4; 95 percent confidence interval, 1.56-7.52). All seven patients with both diseases were diagnosed with ulcerative colitis when older than age 45 years. No differences were found between the two groups in terms of extension of diverticula. CONCLUSIONS Patients with ulcerative colitis show a significantly lower prevalence of colonic diverticulosis, with this finding probably reflecting the motor alterations caused by chronic bowel wall inflammation. In the patients affected by ulcerative colitis with late onset of the disease, the reduced prevalence of colonic diverticulosis is not evident.