Seasonal variations in the prevalence of mental disorders in general population surveys have rarely been noted. Using a representative sample of the Dutch population, the authors studied the seasonality of 1-month mental disorders and gender-by-season and age-by-season interactions. Data were obtained from the Netherlands Mental Health Survey and Incidence Study, a psychiatric epidemiologic study in which 7,076 adults aged 18-64 years were interviewed (1996) using the Composite International Diagnostic Interview. Seasonal statistical differences were not found for the category "any diagnosis", although prevalence was higher in winter (18.1%) and lower in summer (16.0%). Seasonal variations were also not found for the main categories "mood disorders", "anxiety disorders", "substance-use disorders", "eating disorders", and "schizophrenia". Some differences were found for individual disorders, mostly anxiety. Panic disorder was more frequently reported in winter than in summer, generalized anxiety disorder more frequently in winter than in other seasons, and obsessive-compulsive disorder more frequently in autumn than in summer. Only a few gender and age differences in seasonal variation were found. The authors conclude that there are only limited seasonal variations in mental disorders in general population studies, at least in countries with a mild maritime climate. For reliable estimation of the prevalence of some disorders, these findings have implications for the design of large-scale population studies.