Problem solving relies on such abilities as decision-making, planning, initiation and hypothesis testing. Although problem-solving deficits have been consistently reported in depression, the specific nature of these deficits is not fully elucidated. In order to assess and isolate cognitive processes underlying problem-solving impairments in depression, depressed patients and normal controls were evaluated with the modified version of the Wisconsin Card Sorting Test (WCST) and the California Card Sorting Test (CCST). The California Card Sorting Test, unlike the modified WCST, provides several different measures of concept generation, concept identification and concept execution. Compared with controls, depressed patients did not show any deficits on all the measures of the modified WCST. In contrast, depressed patients evidenced mild impairment on the CCST with a specific deficit on concept generation but no major problems in concept identification and concept execution. The deficit in concept generation may be rooted in multiple factors such as hypothesis-testing deficits, a loss of cognitive flexibility and a conservative style of response. Since a positive relation between problem-solving deficits and the mean duration of the depressive episode was observed, problem-solving abilities might be predictive of poorer outcome in patients with unipolar affective disorders.