A group of 355 children who were seen after a first febrile convulsion at the Kaiser Foundation Foundation Hospitals in Southern California from 1970 to 1975 were randomly assigned to three treatment groups-daily phenobarbital, "intermittent" phenobarbital given at the onset of fever, and no phenobarbital. We found that 42% had a relative with a febrile seizure and 16% a relative with an afebrile convulsion. 13% had seizures which were either lateralized or longer than ten minutes. Parents were unaware of the fever prior to the seizure in about 30% of the cases. In 81% the preseizure duration of fever was less than 24 hours. The mean follow-up was 28 months, with a range of 6 to 70 months. There was no significant difference in the recurrence rate between children receiving "intermitent" as compared with no phenobarbital. The recurrence rate in children receiving daily phenobarbital was significantly decreased compared to either of the other two groups. Severe recurrent febrile seizures occurred in no children on daily phenobarbital and in 4.4% of the children receiving either intermittent or no phenobarbital. Parental resistance, compliance, and reversible hyperactivity were the main problems encountered with the continuous phenobarbital regimen.