BACKGROUND & AIMS: The hygiene hypothesis (a lack of childhood exposure to microorganisms increases susceptibility to allergic diseases by altering immune development) has been proposed as an explanation for the increasing incidence of inflammatory bowel disease (IBD). However, there are few data on the relationship between oral hygiene and development of IBD, and study results have been inconsistent. We investigated the association between poor oral health and risks of IBD, ulcerative colitis (UC), and Crohn's disease (CD). METHODS: We performed a population-based cohort study of 20,162 individuals followed for 40 years (from 1973 to 2012). Residents of 2 municipalities of Uppsala County, Sweden (N = 30,118), 15 years or older, were invited, and among them 20,333 were examined for tooth loss, dental plaques, and oral mucosal lesions at the time of study entry. Other exposure data were collected from questionnaires. Patients who later developed IBD (UC or CD) were identified by international classification codes from Swedish National Patient and Cause of Death Registers. Cox proportional hazards regression was used to estimate hazard ratios for IBD, UC, and CD. RESULTS: From National Patient and Cause of Death Registers, we identified 209 individuals who developed IBD (142 developed UC and 67 developed CD), with an incidence rate of 37.3 per 100,000 person-years. We found an inverse relationship between poor oral health and IBD, especially in individuals with severe oral problems. Loss of 5-6 teeth of the 6 teeth examined was associated with a lower risk of IBD (hazard ratio, 0.56; 95% confidence interval, 0.32-0.98). Having dental plaques that covered more than 33% of tooth surface was negatively associated with CD (hazard ratio, 0.32; 95% confidence interval, 0.10-0.97). CONCLUSIONS: In a population-based cohort study of more than 20,000 people in Sweden, we associated poor oral health with reduced risk of future IBD.