Objective: Depression is associated with poor social problem solving, and psychotherapies that focus on problem-solving skills are efficacious in treating depression. We examined the associations between treatment, social problem solving, and depression in a randomized clinical trial testing the efficacy of psychotherapy augmentation for chronically depressed patients who failed to fully respond to an initial trial of pharmacotherapy (Kocsis et al., 2009). Method: Participants with chronic depression (n = 491) received cognitive-behavioral analysis system of psychotherapy (CBASP; McCullough, 2000), which emphasizes interpersonal problem solving, plus medication; brief supportive psychotherapy (BSP) plus medication; or medication alone for 12 weeks. Results: CBASP plus pharmacotherapy was associated with significantly greater improvement in social problem solving than BSP plus pharmacotherapy, and a trend for greater improvement in problem solving than pharmacotherapy alone. In addition, change in social problem solving predicted subsequent change in depressive symptoms over time. However, the magnitude of the associations between changes in social problem solving and subsequent depressive symptoms did not differ across treatment conditions. Conclusions: It does not appear that improved social problem solving is a mechanism that uniquely distinguishes CBASP from other treatment approaches. (Contains 4 tables and 1 figure.)