Objective: This study examines sudden gains (SGs), or rapid improvements in symptoms, among adults in treatment for depression in a partial hospitalization program (PHP). This study identifies the proportion of people who experience SGs in a PHP, when SGs occur in treatment, and the association of SGs with outcomes at the end of treatment. Method: The sample included 664 adults consecutively admitted to a PHP program for treatment of depression. Patients were administered the 24-item Behavior and Symptom Identification Scale and the short form of the Quality of Life Enjoyment and Satisfaction Questionnaire. We conducted t tests, multiple regressions, and generalized estimating equations (GEE). Results: Over 40% of the sample experienced SGs, and most of those who experienced SGs did so by the 2nd week of treatment. SGs were associated with significantly greater improvement in depression and quality of life scores at the end of treatment. Results of the GEE indicate that although depression scores significantly decreased for the entire sample, the SG group improved to a greater degree than the group without SGs. Conclusions: The proportion of SGs among people in a PHP is similar to proportions found in traditional outpatient psychotherapy, suggesting that a subset of people receiving psychotherapy may be predisposed to SGs. The timing of SGs in a PHP suggests that SGs are associated with the dose of treatment received. Future research should focus on identifying people predisposed to SGs, as well as the mechanisms by which SGs occur. (Contains 3 tables and 1 figure.)