Among a large workplace population interviewed over a year, current symptoms were assessed using the Hopkins Symptom Checklist (HSCL). Variation in symptoms by date of assessment was observed among the 314 women but not the 1556 men. Among women, symptoms were greatest during late autumn and winter, and significant inverse correlations were found between available daylight at the time of assessment and standard symptom dimensions of anxiety and somatisation, as well as an expanded mood scale more inclusive of depressive symptoms within the check-list. The amplitude of the seasonal effect was such that the prevalence of female 'cases', as defined by HSCL criteria, was twice as high during winter than during the rest of the year. The data are consistent with a role for light-dark exposure in eliciting or synchronizing annual mood rhythms. The sex-by-season interactions may contribute to the sex differences in overall prevalence of depression.