Most females with seasonal affective disorder (SAD) exhibit atypical vegetative symptoms such as overeating, and weight gain when depressed. The serotonin 2C receptor (5-HT(2C)) plays a key role in control of appetite and satiety. A 5-HT(2C) Cys 23 Ser substitution, coded for by a single nucleotide polymorphism (Cys 23 Ser) within the 5-HT(2C) gene, has been shown to influence 5-HT(2C) function. We hypothesized that Cys 23 Ser influences weight regulation in females with SAD. Two independent samples from Austria (162 females with SAD, 119 controls), and Canada (90 females with SAD, 42 controls) were genotyped for Cys 23 Ser. Influence on weight regulation was analyzed within patients with atypical features. In Austrians, genotype distribution differed between patients and controls (p=0.044) and Cys 23 Ser was associated with weight (p=0.039), body mass index (BMI; p=0.038), and seasonal appetite change (p=0.031). All values were highest in Cys/Cys, intermediate in Cys/Ser, and lowest in Ser/Ser carriers. In Canadian patients, Cys 23 Ser was associated with minimum lifetime BMI (p=0.046), with lowest values in Ser/Ser carriers. Our data provide evidence that Cys 23 Ser mediates severity of weight regulation disturbances in females with SAD, and the gene-dose effect-like differences suggest a direct functional role of Cys 23 Ser in the behavioral regulation of body weight.