The phenomenon of seasonality of birth in schizophrenia is important in the study of the etiology of this mental disorder because it helps to give directions for further research. Patients' hospital files from 1981 to 1991 of two of the largest hospitals with psychiatric wards in Taiwan were reviewed, and dates of birth collected on 3346 patients diagnosed with schizophrenia. After adjusting for the variations of the total monthly births in the population, an Auto-Regressive Integrated Moving Average (ARIMA) model was applied. Results support a seasonality phenomenon and indicate a disproportional excess of births in schizophrenia in the cold months (Nov. to Feb.) compared to the hot months (May to Aug.). These findings are compatible with many other studies in other countries and climates. Further investigations of season-related environmental factors in the etiology of schizophrenia are recommended.