We used the Structured Interview for DSM-III Personality Disorders to diagnose DSM-III personality disorders systematically in 55 patients with obsessive-compulsive disorder in the active-treatment cell of a controlled trial of clomipramine hydrochloride. Patients with a cluster A personality disorder had significantly higher obsessive-compulsive disorder severity scores at baseline, and the number of personality disorders was strongly related to baseline severity of obsessive-compulsive disorder symptoms. At the conclusion of the 12-week study, we found no significant difference in treatment outcome with clomipramine between those patients with at least one personality disorder and those with no personality disorders. However, the presence of schizotypal, borderline, and avoidant personality disorders, along with total number of personality disorders, did predict poorer treatment outcome. These variables were strongly related to having at least one cluster A personality disorder diagnosis, which was also a strong predictor of poorer outcome. Implications of these findings are discussed.