Psychiatric disorder has long been considered to have seasonal variation. The studies to date have frequently suffered from small samples and imprecise terminology, and the results have been inconclusive. This study has attempted to overcome these difficulties by examining hospital admissions to all facilities in the province of Ontario for a six-year period, with each year carefully divided into seasons. Statistically significant seasonal variation, with peaks in the spring and fall, was found overall for neurotic and endogenous depression. Alcoholism also showed a spring peak. No other diagnoses, overall, showed seasonality, although personality disorders, drug addictions, and transient situational disturbances exhibited trends similar to neurotic depression for certain age and sex groups. The findings are discussed in terms of their clinical and research significance.