One hundred and eighty-six consecutive children aged between six and 72 months admitted to the Manchester Children's Hospitals with a febrile convulsion in the first year of life, a complicated febrile convulsion, or more than one febrile convulsion within two years, were allocated randomly to one of three groups who were offered rectal diazepam in the event of a subsequent prolonged febrile convulsion, or prophylactic treatment with sodium valproate or phenobarbitone. Over-all risk of recurrence was 30 per cent and adequate prophylactic treatment did not lessen this risk. Side-effects in 24 per cent of the valproate group and 61 per cent of the phenobarbitone group did not justify the use of prophylactic treatment.