In a prospective study, 43 children between three months and 16 years of age and suffering from an acute infection of the lower urinary tract, were treated for either three or ten days with 4/16 mg trimethoprim/sulphadiazine/kgBW/day in two doses. Twenty-three were allotted to treatment for three days, whereas 20 were treated for ten days. Irrespective of the duration of therapy, the urine of all patients was sterile when urinary cultures were made three to seven days after the cessation of therapy. An early recurrence within the two months following the completion of treatment occurred in two children in each treatment group. In no case of recurrence was the organism resistant to trimethoprim/sulphadiazine. During a mean follow-up period of 11 months, 21.7% of the children treated for three days and 35% of those treated for ten days experienced a recurrence. The results suggest that children with an uncomplicated lower urinary tract infection can be successfully treated with a three-day course of trimethoprim/sulphadiazine in a conventional dosage.