We evaluated the risks of seizures and other neurological events following diphtheria-tetanus-pertussis (DTP) immunization for 38,171 Tennessee Medicaid children who received 107,154 DTP immunizations in their first 3 years of life. There were 2 children with encephalitis; both had disease onset more than 2 weeks following DTP immunization. There were 277 children who had febrile seizures, 42 with afebrile seizures, and 37 with seizures associated with other acute neurological illness (acute symptomatic). The risk of febrile seizures in the 0 to 3 days following DTP immunization (n = 6) was 1.5 (95% confidence interval, 0.6 to 3.3) times that of the control period 30 or more days following DTP immunization. There was no evidence that in the 0 to 3 days following DTP immunization the risk of afebrile seizures (n = 1) or acute symptomatic seizures (n = 0) was increased. No child who was previously normal without a prior history of seizures had a seizure in the 0 to 3 days following immunization that marked the onset of either epilepsy or other neurological or developmental abnormality.