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To date, little attention has been paid to changes in neuropsychological function in seasonal affective disorders (SAD). In this study, we investigated the performance of 30 patients with SAD on a wide range of cognitive variables before and after 2 weeks of light treatment with either white or placebo red light, as well as later in the summertime. Performance of subjects with SAD on neuropsychological tests was compared with a group of 29 age- and education-matched healthy control subjects. The most consistent deficits associated with SAD were on tests of cognitive failures, visual memory, and visual-construction skills. In contrast to specific bright light effects on psychiatric measures, reports of cognitive failures did not change with either light treatment. Visual memory and constructional deficits responded nonspecifically to treatment with either white or the presumed placebo red light. Surprisingly, visual memory deficits were seen again in the summer, at a time when mood, cognitive failures, and other cognitive functions appeared at normal levels. These data suggest that cognitive functioning is affected by SAD. In addition, light treatment may have differential effects on mood and cognition.