The aim of this study was to compare the prevalence of Personality Disorders assessed by Structured Clinical Interview for Axis-II in 155 inpatients diagnosed with Unipolar Disorder vs inpatients with Bipolar Disorder (39). The most frequent Axis II diagnoses among Unipolar inpatients were Borderline (31.6%), Dependent (25.2%), and Obsessive-Compulsive (14.2%) Personality Disorders. Among Bipolar inpatients, the most prevalent personality disorders were Borderline (41%), Narcissistic (20.5%), Dependent (12.8%), and Histrionic disorders (10.3%). Using chi squared analysis, few differences in distribution emerged between the two groups: Unipolar patients had more recurrent Obsessive-Compulsive Personality Disorder than Bipolar patients (chi(1)2=6.24, p<.005). Comorbid Narcissistic Personality Disorder was significantly more frequent in the Bipolar than in the Unipolar group (chi(1)2=6.34, P<.01). Considering the three clusters (DSM-IV classification), there was a significant difference between the groups, Cluster C (fearful, avoidant) diagnoses being more frequent in the Unipolar than in the Bipolar group (48.4% vs 20.5%, respectively). Cluster B (dramatic, emotionally erratic) diagnoses were found more frequently in patients with Bipolar Disorders (71.8% vs 45.2% in Unipolar patients, chi(2)2=10.1, p<.006). The differences in the distribution and prevalence of Personality Disorders between the two patient groups are discussed.