The etiology of diverticulitis is poorly understood. The long-held belief that constipation and low-fiber diet are risk factors for diverticulosis has recently been challenged by studies that suggest that more frequent bowel movements predispose to diverticulosis. We aim to prospectively explore the association between bowel movement frequency and incident diverticulitis.We studied participants of the Nurses' Health Study (NHS) and Health Professional Follow-up Study (HPFS). Participants' medical history, lifestyle factors and diet were used in Cox proportional hazards regression models to estimate multivariable-adjusted hazard ratios(HRs) and 95% confidence intervals(CI).In the NHS during over 24 years of follow-up encompassing 1,299,922 person-years, we documented 5,214 incident cases of diverticulitis, and in HPFS over 14 years encompassing 368,661 person-years of follow-up, we documented 390 incident cases of diverticulitis. We observed an inverse association between the frequency of bowel movements and risk of diverticulitis. In NHS, compared to women who had daily bowel movements, those with more than once daily bowel movements had a HR of 1.30 (95%CI 1.19, 1.42) and those with less frequent bowel movements had a HR of 0.89 (95%CI 0.82, 0.95; p-trend<0.0001). In HPFS, the corresponding HRs were 1.29 (95%CI 1.04, 1.59) and 0.61 (95%CI 0.36, 1.03; p-trend=0.003). The association between bowel movements and diverticulitis was not modified by categories of age, BMI, physical activity, laxative use or fiber intake.More frequent bowel movements appear to be a risk factor for subsequent diverticulitis both in men and women. Further studies are needed to understand the potential mechanisms that may underlie this association.