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Objective: To use a family genetic study to evaluate familial risk of obsessive compulsive disorder (OCD) and common comorbid illnesses in first-degree relatives of pediatric-onset probands with primary OCD. Method: One hundred and thirty youth with OCD and their 133 siblings and 241 parents and 49 pediatric controls were directly evaluated along multiple domains including psychopathology using structured diagnostic interviews and clinical corroboration. Results: Rates of anxiety, mood, disruptive behavior, and tic disorders were markedly elevated in the probands while rates in siblings were elevated at rates between the probands and controls. Twenty six percent of first-degree relatives had clinical OCD, 9% had chronic tics or Tourette's disorder, and 21% met criteria for ADHD. Conclusion: Rates of familial transmission of OCD and common comorbid illnesses were significantly higher in our pediatric-onset probands than rates reported in the literature in relatives of those with adult-onset OCD.