Parents of autistic probands with high and low rates of repetitive behaviors were compared for rates of obsessive-compulsive traits and disorder. The rate of repetitive behaviors was assessed using the Autism Diagnostic Interview-Revised (ADI-R) in 176 autistic probands from 57 multiplex families. Obsessive-compulsive disorder (OCD) in parents was determined by direct interview using a parental history questionnaire, with screening for obsessive-compulsive traits using the Yale-Brown Obsessive-Compulsive Scale checklist. Children who had high total scores on the repetitive behavior domain of the ADI-R were significantly more likely to have one or both parents with obsessive-compulsive traits or disorder compared with children who had low total scores on this domain. Children with high scores on D1/D2 of the ADI-R (narrow restricted interests and rituals) were significantly more likely to have one or both parents with OCD, especially fathers, than those with low D1/D2. The occurrence of obsessive-compulsive traits or disorder in parents of autistic children in multiplex families is significantly more likely if autistic children have a high occurrence of repetitive behaviors. Dichotomizing autistic probands by severity and type of repetitive behaviors (circumscribed interests and compulsive rituals) may yield more homogenous groups, which could be helpful in genetic linkage studies.