OBJECTIVE: Microscopic colitis is a common cause of chronic watery diarrhea among the elderly. Although the prevalence of celiac disease appears to be higher in patients with microscopic colitis, the relationship between dietary gluten intake and risk of microscopic colitis among individuals without celiac disease has not been explored. METHODS: We conducted a prospective study of 160,744 US women without celiac disease enrolled in the Nurses' Health Study (NHS) and the NHSII. Dietary gluten intake was estimated using validated food frequency questionnaires every 4 years. Microscopic colitis was confirmed through medical records review. We used Cox proportional hazard modeling to estimate the multivariable-adjusted hazard ratio (HR) and 95% confidence interval (CI). RESULTS: We documented 219 incident cases of microscopic colitis over more than 20 years of follow-up encompassing 3,716,718 person-years (crude incidence rate: 5.9/100,000 person-years) in NHS and NHSII. Dietary gluten intake was not associated with risk of microscopic colitis (Ptrend = 0.88). Compared to individuals in the lowest quintile of energy-adjusted gluten intake, the adjusted HR of microscopic colitis was 1.18 (95% CI: 0.77-1.78) for the middle quintile and 1.03 (95% CI: 0.67-1.58) for the highest quintile. Additional adjustment for primary dietary sources of gluten including refined and whole grains did not materially alter the effect estimates (All Ptrend ≥ 0.69). The null association did not differ according to lymphocytic or collagenous subtypes (Pheterogeneity = 0.72) and was not modified by age, smoking status, or body mass index (All Pinteraction ≥ 0.17). CONCLUSIONS: Dietary gluten intake during adulthood was not associated with risk of microscopic colitis among women without celiac disease.