Acetaminophen and low doses of diazepam were evaluated for the prevention of recurrences of febrile seizures in a placebo-controlled, double-blind trial. Children after their first febrile seizure were assigned to receive either one dose of rectally administered diazepam, and then, after 6 hours, oral doses of 0.2 mg/kg three times a day for the first 2 days if the fever stayed greater than 38.5 degrees C, or a placebo similarly during forthcoming febrile episodes. In addition, each febrile episode was randomly assigned to be treated with acetaminophen or the placebo. Thus four groups were examined for 2 years: patients receiving two kinds of placebo, patients receiving diazepam and a placebo, patients receiving acetaminophen and a placebo, and patients receiving both diazepam and acetaminophen. Of a total of 180 patients (102 boys), 161 were followed for the 2-year period and 157 were seen at the last outpatient examination: 80 in the diazepam group and 77 in the placebo group. The final analysis of the efficacy of the drugs was made on the basis of the data from 153 patients who had had at least one febrile episode during follow-up. There were 641 fever events during this period, and 38 children (21.1%) had 55 recurrences of febrile seizures. Acetaminophen had no effect on the recurrence rate. Seizures recurred at least once in 21 patients (28.4%) receiving diazepam and 17 (21.5%) receiving a placebo (p = 0.4138, log-rank test). The combination of antipyretic agents with anticonvulsant medication did not reduce the recurrence of febrile seizures. Our results show that low doses of acetaminophen or diazepam or both are ineffective for preventing febrile seizures.