The aim of the study was to assess the prevalence of personality disorders in a group of outpatients with bipolar I disorder. The Structured Clinical Interview for DSM-III-R Personality Disorders (SCID-II) was administered to 90 bipolar outpatients who met the DSM-III-R criteria and 58 control subjects. Of the patients and controls, 47.7% and 15.5%, respectively, had at least one personality disorder. At least one personality disorder in clusters A, B, and C and obsessive-compulsive, paranoid, histrionic, and borderline personality disorders were significantly more prevalent in bipolars. Suicide attempts were more frequent in patients with a history of personality disorder.