Comparison of the effects of psychiatric day hospital programs between homogenous clinical groups is an important issue that requires more attention. One group of day hospital patients who have not been included in most studies are people with cluster B personality disorders. The purpose of this study was to compare clinical and social participation outcomes in three groups of individuals treated in a psychiatric day hospital: patients with psychotic disorders, patients with mood and anxiety disorders, and patients with cluster B personality disorders. A pre-experimental, pre-test post-test design was used. During the first and last week of treatment, as well as 6 months after discharge, 20 participants in each group completed questionnaires on severity of symptoms, distress, accomplishment, satisfaction with social participation, and self-esteem. During the intervention, there was significant improvement in all groups on all variables, except for self-esteem in people with psychotic disorders, which remained stable. The patients with psychotic disorders showed significantly less improvement than the two other groups in severity of symptoms, distress, and self-esteem. Following discharge, the degree of change was comparable in the three groups on all variables based on between-groups analyses. However, based on within-group analyses, patients in the mood and anxiety disorders group continued to show significant improvement over time after discharge on self-esteem, accomplishment, and satisfaction with social participation, while no significant changes were seen in the other two groups. Although all three clinical groups made significant gains during their participation at the day hospital and maintained these gains after discharge, those with mood and anxiety disorders benefited the most from their day hospital experience.