To report descriptive epidemiological information on obsessive-compulsive disorder (OCD) in an unselected birth cohort of 930 males and females, aged 18 years. An epidermiological study of the prevalence of self-reported OCD at age 18, and a longitudinal analysis of the prospective predictors of OCD. Using the Diagnostic Interview Schedule, the authors found a 1-year prevalence rate of 4%, with a male-female ratio of 0.7:1. The majority of OCD cases met criteria for a comorbid disorder, most commonly depression (62%), social phobia (38%), and substance dependence (alcohol 24%, marijuana 19%). Data collected on the sample from birth to age 18 years indicated that many childhood risk factors theorized in the literature did not predict OCD in this sample. However, a history of depression and substance use were prospective risk factors for OCD.