There has been much discussion in Japan regarding the reduction of psychiatric beds. For effective healthcare planning, reliable forecasting is important. The purpose of this study was to predict the number of future schizophrenic inpatients using quantitative methodology. Data was obtained from a survey of schizophrenic inpatients conducted annually at the end of March by the Niigata Prefecture from 1974 to 2003. The numbers of schizophrenic inpatients in different age groups over a long period of time were used in a precise time-series analysis to establish trends. Then these past trends were used to forecast inpatient numbers for future years. The pattern of ascents and declines of each inpatient group stratified by age appeared to be duplicated by the next older age group 10 years later. The numbers of inpatients with schizophrenia in 2013 and 2023 are projected to be 78.5% and 56.7% of the number of patients in 2003, respectively. By 2033, the number is forecast to decline to 41.0% of the number in 2003. This study forecasts that inpatients with schizophrenia will decrease substantially over the next several decades. Policy should be designed to reflect this trend.