The aim of this study was to identify environmental factors that might correlate with the admission rate of patients with major psychiatric disorders. During the years 1988-90, 393 consecutive admissions (119 unipolar depressed patients, 211 schizophrenic patients, and 63 bipolar depressed patients) were monitored. Correlations were calculated between the mean daily admission rate for each month and monthly photoperiod, rate of change in photoperiod, mean temperature, mean relative humidity, and mean barometric pressure. It was found that the admission rate of bipolar depressed patients negatively correlated with monthly photoperiod, which means that during winter the admission rate of these patients increased.