A cross-over comparison study of exposure, in the evenings only, to bright versus dim light was carried out on nine female patients with seasonal affective disorder. A significant antidepressant effect of the bright lights was shown. No consistent observable effects were produced by the dim lights. These results support earlier studies demonstrating the efficacy of bright light given morning and evening. The antidepressant effect of light is not mediated by sleep deprivation, and the early morning hours are not crucial for a response.