The Monitoring System for Adverse Events Following Immunization (MSAEFI) has collected data from the public sector nationwide on adverse events occurring during the 4-week period following administration of vaccine. From 1979 to 1984, 6483 reports were received. Although rates of reporting have increased throughout the 6-year period, increases were larger for less serious events (209%) than for more serious events (53%). The MSAEFI data have been used to evaluate risk factors for adverse events following immunization, will continue to provide information on the safety of both current and future vaccines and may identify other factors that increase the risks of adverse events following immunization.