The Monitoring System for Adverse Events Following Immunization became fully operational in late 1978 in all 50 States, New York City and Washington, D.C. This system compiles information on adverse events temporally associated (within four weeks) to the administration of a vaccine. A total of 2,062 reports of adverse events following diphtheria and tetanus toxoids and pertussis vaccine (DTP) were received between 1979 and 1982. The number of reports received in 1979 was 387, increasing to 707 reports in 1982. The increase is predominantly in local reactions and fever. The number of persons with serious illnesses temporally associated with vaccine remained relatively constant over the four year period. The overall rate of reported adverse events following DTP (70.8 per million doses administered in the public sector) was about twice that of Td (33.5) or DT (38.4); however it was not substantially different from those following measles- or rubella-containing vaccines. Those who had convulsions (whether febrile or non-febrile) following receipt of DTP vaccine were substantially more likely to have had a personal history of convulsions than those who had a non-neurologic adverse event following DTP (p less than 0.0001). This information, along with data from the literature, has led to recent recommendations by advisory groups in the United States that any infant or child with a personal history of convulsions should be thoroughly evaluated before initiating or continuing immunization with pertussis vaccine.