To determine the seasonal variation of childhood infectious diseases in Taiwan, the incidence rates of chickenpox, mumps and rubella were studied by analyzing patient's records collected from pediatric clinics in northern, central, southern and eastern Taiwan from January 1, 1993 to December 31, 1993. The study revealed that a chickenpox outbreak was most likely to occur in autumn, followed by a second peak in spring, and the lowest rate in summer. In this study, the highest monthly incidence of chickenpox was 540 cases per 100,000 population. The highest incidence of mumps occurred in the spring, the second highest in autumn, and the lowest during summer. Cases of rubella occurred in the spring, summer and autumn, but not in the winter. The data on chickenpox from this study revealed a similar seasonal pattern every year to that of the Department of Health of the National Quarantine Service on Taiwan. The chickenpox virus is known to be relatively sensitive to elevations in ambient temperature and is characterized by its occurrence mainly in slightly humid seasons.