Obsessive-compulsive disorder (OCD) is a heterogeneous disorder of unknown etiology. We examined the lifetime history of obsessions, compulsions, and OCD in the first- and second-degree relatives of 35 pediatric probands with OCD and 17 controls with no psychiatric diagnosis. All available first-degree relatives were directly interviewed blind to proband status with two semi-structured interviews. Parents were also interviewed to systematically assess the family psychiatric history of first- and second-degree relatives. Best-estimate lifetime diagnoses were made using all available sources of information. Data were analyzed with logistic regression by the generalized estimating equation method and with robust Cox regression models. The lifetime prevalence of definite OCD was significantly higher in case than control first-degree relatives (22.5% vs. 2.6%, P < 0.05). Compared to controls, case first-degree relatives also had significantly higher lifetime rates of obsessions and compulsions (both P < 0.05). There was no significant difference between case and control second-degree relatives in lifetime rates of OCD. First-degree relatives of case probands with ordering compulsions had a significantly higher lifetime rate of definite and subthreshold OCD than relatives of case probands without ordering compulsions (45.4% vs. 18.8%, P < 0.05). The lifetime prevalence of definite OCD was significantly higher in case first-degree relatives with a history of tics than in case first-degree relatives without a tic history (57.1% vs. 20.9%, P < 0.01). The results provide further evidence that early-onset OCD is highly familial and suggest that two clinical variables are associated with its familial aggregation. © 2004 Wiley-Liss, Inc.