The authors review what has been learned about the causes, symptoms, and treatments of seasonal affective disorder and discuss its relevance to affective illness in general. They point out that seasonal and environmental influences on depression have been themes in writings on affective illness for more than 2,000 years and that there has been a resurgence of interest during the past decade. There appear to be two primary, opposite seasonal patterns of annual depression--winter depression and summer depression--with opposite vegetative symptoms. Seasonal affective disorder is not uncommon. It is important to identify patients with winter depression because they respond to a specific treatment, phototherapy.