Seasonal affective disorder (SAD) and premenstrual dysphoric disorder (PMDD) share many clinical features, and have been associated with brain serotonin dysfunction. Females with SAD frequently fulfil the diagnostic criteria for PMDD. A polymorphism in the serotonin transporter promoter gene (5-HTTLPR) has been associated with SAD. We investigated the role of family history and 5-HTTLPR in female SAD patients with and without PMDD. Forty-four SAD females with, and 43 SAD females without PMDD, were genotyped for 5-HTTLPR. Family history of affective disorders in first degree relatives was assessed. An association between the presence of PMDD and family history (P=0.0029) and 5-HTTLPR long/short allele-heterozygosity (P=0.033) was found in females with SAD. PMDD and SAD may share genetic vulnerability factors, one candidate gene being 5-HTTLPR. The elevated rate of affective disorders in relatives of patients with SAD and PMDD suggests higher genetic vulnerability in this subgroup when compared to patients with SAD alone.