To examine perinatal risk factors for Crohn's disease and ulcerative colitis, the authors analyzed birth records for 257 known case participants delivered from 1924 through 1957 at the University Hospital in Uppsala County, Sweden, and compared them with records for 514 controls delivered at the hospital. The two groups were matched by date of birth, sex, and either maternal age or parity. Eleven study variables were abstracted from standard forms that recorded health events during the pregnancy and the delivery hospitalization. Participants were more likely than controls to have a recorded health event (odds ratio (OR) = 4.4; 95% confidence interval (CI) 3.0-6.4). In a multivariate model, this increased risk was evident for infectious (OR = 3.8; 95% CI 2.6-5.8) and noninfectious (OR = 3.5; 95% CI 2.0-6.3) events. Perinatal health events may have contributed to 40% of the inflammatory bowel disease cases in our study. Infants from families with low socioeconomic status had greater risk of inflammatory bowel disease than did infants from families with high socioeconomic status (OR = 3.0, 95% CI 1.5-6.1). Perinatal health events and low socioeconomic status independently increased the risk of inflammatory bowel disease.