We evaluated the efficacy of diazepam and chloral hydrate given rectally for the prophylaxis of recurrence of febrile seizure. The dose were 0.4 mg/kg for diazepam and 250 mg (for children younger than 3 years old) or 500 mg (for over 3 years old) for chloral hydrate. Another dose was given after an interval of 8 hours if body temperature continued to exceed 38.0 degrees C. Among the 452 patients with febrile seizures who visited our hospital from Jan. 1993 to Jun. 1995, 113 were studied who had at least one febrile episode in the follow-up period that extended over 6 months. These patients were divided into two groups: Group D (72 patients given diazepam) and Group C (41 patients given chloral hydrate). In group D and C, the numbers of febrile episodes were 238 and 167, and those of recurrent seizures 8 (3.8%) and 29 (20.4%), respectively. The recurrences rate was significantly higher in the latter group (p < 0.005). There was no statistical difference as to the mean dosage of diazepam or chloral hydrate between the patients with and without recurrence. The numbers of patients with seizure recurrence were 8 (11.1%) in group D and 12 (29.3%) in group C, being significantly larger in the latter (p < 0.005). Diazepam produced more adverse effects than chloral hydrate did. Thus diazepam suppositories for the prevention of recurrence of febrile seizures were more effective than chloral hydrate suppositories.