In 1.4% of children who experienced a seizure during the first seven years of life, the seizure followed within two weeks of an immunization procedure. We report 40 postimmunization seizures in 39 children enrolled in the Collaborative Perinatal Project. Ten seizures followed diphtheria-pertussis-tetanus (DPT) immunization, and 10 followed measles immunization. All but one of the seizures were associated with fever, often high. Thirty-seven seizures lasted less than 30 minutes. More than half of the children had a personal or immediate-family history of febrile seizures. One of the children had a right focal seizure lasting six hours after DPT immunization and had a significant speech deficit on long-term follow-up. No child developed epilepsy, and results in all children with brief seizures were normal on neurologic and cognitive examination at 7 years of age. Both in clinical presentation and generally benign outcome, these immunization-related seizures closely resemble febrile seizures, which are common in early childhood.