One hundred and forty-five first-degree relatives (89 parents [96%] and 56 siblings [98%]) of 46 children and adolescents with severe primary obsessive compulsive disorder (OCD) were personally interviewed with clinical and structured psychiatric interviews. Parent interviews were scored by a rater blind to proband diagnosis. Thirty percent of probands had at least one first-degree relative with OCD: 25% of fathers and 9% of mothers received this diagnosis. Forty-five percent of fathers and 65% of mothers received one or more other psychiatric diagnoses. The increased familial rate of OCD over that expected from a general population, and over that found in parents of conduct disordered patients, is consistent with a genetic factor in OCD. Presenting obsessive compulsive symptoms of probands and their parents were usually dissimilar, arguing against any simple social or cultural transmission.