The study of the genetic basis of seasonal affective disorder (SAD), a condition where depressions in fall and winter alternate with nondepressed periods in the spring and summer, has recently received attention. The data on the genetics of seasonal affective disorders are of three types: 1. Familiality: Studies on the prevalence of psychiatric disorders among relatives of patients with SAD suggested a familial contribution to the development of SAD; 2. Heritability: A survey of a cohort of twins showed that genetic effects exert a global influence across a variety of behavioral traits and accounted for at least 29% of the variance in seasonality in men and women; 3. Molecular genetic research: two genetic variants related to serotonergic transmission, the 5-HTTLPR and the 5-HT2A-1438G/A gene promoter polymorphisms, are associated with SAD; the former but not the latter polymorphism is related to seasonality. Future research may clarify the role of different genes in the development of SAD.