To evaluate the risks of seizures and other neurologic events following measles-mumps-rubella (MMR) or measles-rubella (MR) immunization, a retrospective cohort study was conducted among 18,364 Tennessee children enrolled in Medicaid who received MMR or MR immunizations in their first 3 years of life. One hundred children had seizures at some time between immunization and 36 months; there were no encephalopathies during this period. Four children had febrile seizures in the 7 through 14 days following MMR or MR immunization compared with 72 in the interval 30 or more days following MMR or MR immunization yielding a relative risk (95% confidence interval) of 2.1 (0.7 to 6.4). Although not statistically significant, this increase in febrile seizures in the 7- through 14-day interval following MMR immunization is coincident with the occurrence of fever following MMR immunization and is consistent with reports of other investigators.