A partial hospitalization program was developed for youth with moderate to severe psychiatric disorders. The objectives of this study were to prospectively assess changes from admission to discharge in the participants' clinical symptoms and psychosocial functioning, as well as the maintenance of any treatment gains at 3-month follow-up. In this naturalistic treatment study, 55 youth completed both the youth partial hospitalization program and the study. Diagnosis at admission was established by a psychiatrist based on a clinical assessment and a structured diagnostic interview. Clinician-rated and self-report measures of symptoms and psychosocial functioning were administered at admission, discharge, and follow-up to track changes over time. Clinically and statistically significant improvements were found in both symptoms and psychosocial functioning from admission to discharge on all study measures. Furthermore, these gains were maintained at 3-month follow-up. The results suggest that partial hospitalization programs can be an effective modality in treating youth with moderate to severe psychiatric disorders.