Objective: There are few studies regarding the prevalence of seasonal variation in mood among children and adolescents. The main objective of this study was to estimate the prevalence of self-reported depressive mood during the winter season among Swedish adolescents and to investigate gender differences. Another aim was to analyze the factor structure and internal consistency of the Kiddie SPAQ (K-SPAQ), a pediatric version of the Seasonal Pattern Questionnaire (SPAQ). Method: All students 17 to 18 years old, registered in the second year of senior high school in Falun, a district in central Sweden, were screened with the K-SPAQ in January 2003 (response rate 87.3%, n = 756). Results: The prevalence of self-reported depressive mood during the winter season was estimated at 20.1% (n = 151/751) and was higher among girls (25.5%) than boys (13.8%). Approximately 8% reported more severe depressive symptoms. Depressive mood during the summer was rare (0.1%, n = 1/751). Factor analysis of the General Seasonal Score items in the K-SPAQ revealed a two-factor structure. A Cronbach's [alpha] of 0.87 demonstrated a good internal consistency. Conclusions: Depressive symptoms during autumn and winter were common among Swedish senior high school students, especially among girls. This is probably an underdiagnosed condition among adolescents that ought to receive more attention from the health and school authorities. (Contains 1 figure and 3 tables.)