A randomized double-blind study was carried out comparing single daily dose phenobarbital plus antipyretic instruction to a placebo plus antipyretic instruction to prevent a recurrent seizure following an initial simple febrile seizure. Parents of 138 consecutive children presenting to an emergency room with a first simple febrile convulsion received verbal and written instructions about fever control. Seventy-nine then agreed to participate in this study. Children were randomized to receive either placebo with riboflavin tracer (n = 40) or phenobarbital 5 mg/kg in a single daily dose with a riboflavin tracer (n = 39) for 12 months or until another seizure occurred. Urine fluorescence for riboflavin was used to monitor compliance in all patients. Serum phenobarbital levels were obtained at each follow-up visit and averaged 1.4 mg/dl throughout the study. The significant difference (P less than 0.02) in the incidence of recurrent seizures between patients receiving phenobarbital (2/39) and those receiving placebo (10/40) suggests that a single daily dose of phenobarbital is more effective than counseling parents about antipyretic therapy in preventing recurrent seizures following an initial febrile seizure.