OBJECTIVE Colonic diverticulosis, although usually asymptomatic, sometimes causes diverticular hemorrhage. Studies about risk factors, other than nonsteroidal anti-inflammatory drugs, for colonic diverticular hemorrhage are limited. We conducted the present study to elucidate their significance as a risk factor. METHODS Colonic diverticulosis was found in 1,753 patients and diverticular hemorrhage in 44 patients among 9,499 total colonoscopy examinees at the authors' institutions between September 1995 and December 2005. After reviewing their clinical features, we chose two controls for each case with diverticular hemorrhage matched for age, gender, and the location of diverticulosis. We evaluated the effects of comorbidities (hypertension, hyperlipidemia, diabetes mellitus, cerebrovascular disease, ischemic heart disease), habits (alcohol, smoking), and medications, including nonsteroidal anti-inflammatory drugs, by using conditional logistic regression analysis. RESULTS There were no significant differences between patients with diverticular hemorrhage and those with nonbleeding diverticulosis regarding age (67 +/- 13 vs. 64 +/- 11 years) or gender ratio (male/female ratio: 36/8 vs. 1,237/472). As for location, the proportion of bilateral diverticulosis was larger among patients with hemorrhage (43 vs. 22 percent). In the case-control study, nonsteroidal anti-inflammatory drugs (odds ratio, 15.6; 95 percent confidence interval, 1.1-214; P = 0.04), hypertension (odds ratio, 6.6; 95 percent confidence interval, 2.1-20.5; P = 0.0011), and aspirin and/or other anticoagulant (odds ratio, 3; 95 percent confidence interval, 1.04-8.6; P = 0.042) were shown to be significant risk factors by multivariate analysis. CONCLUSIONS Hypertension, nonsteroidal anti-inflammatory drugs, and anticoagulants, including aspirin, are independent risk factors for colonic diverticular hemorrhage.