The present study tested the potential association between the 3111C/T polymorphism of the CLOCK gene and seasonal variations in mood and behavior. A total of 297 Korean college students were genotyped for the CLOCK polymorphism and the seasonal variation was evaluated using the Seasonal Pattern Assessment Questionnaire (SPAQ). The seasonality scores were not different between CLOCK gene variants (P > 0.05). Comparison between seasonals (syndromal plus subsyndromal seasonal affective disorder according to SPAQ) and non-seasonals found no significant difference in frequencies of genotypes (P > 0.05). These findings suggest that the CLOCK polymorphism does not play a major role in susceptibility to seasonal variations in a Korean population.