In a prospective study of children with an acute infection of the lower urinary tract, the effectiveness of a 3-day course of cephalexin, 25-50 mg/kg body weight and day was compared with that of a 10-day course of nitrofurantoin, 3-4 mg/kg/day. 19 children were allotted to treatment with cephalexin and 24 were treated with nitrofurantoin. The immediate cure rates were 90% and 96%, respectively. Two relapses were noted in the cephalexin group and 1 in the nitrofurantoin group. During a mean follow-up period of 7-8 months 2 of the cephalexin treated patients and 4 patients treated with nitrofurantoin had a reinfection. No side effects were noted in either of the treatment groups. The results suggest that treatment with cephalexin for 3 days is a reasonable alternative in children with an acute lower urinary tract infection when commonly used medications for one reason or another are less well tolerated.