In children, the site of urinary tract infection (acute pyelonephritis or cystitis) cannot usually be accurately determined from the clinical presentation. The severity of the urinary tract infection (risk of renal scars) is best correlated with its estimated degree of tissue penetration clinically (fever, general condition) and on laboratory tests (sedimentation rate, C-reactive protein). The duration of parenteral antibiotic therapy, especially in children (taking account of difficult venous access and the cost of hospitalization) needs to be specified beyond the initial period required for sterilization of the urine (usually less than 48 h). We conducted a study in children older than one year to compare the efficacy and tolerance of two treatment regimens for urinary tract infection with tissue penetration: cefotaxime 100 mg/kg/d in four divided iv doses for 14 days (group I) and amoxycillin/clavulanate 100 mg/kg/d in four divided iv doses for seven days with conversion to the oral route at a dosage of 50 mg/kg/d for seven days (group II). The randomised protocol included ten patients in each group, comparable with respect to sex, age and history. Clinical efficacy (time until the patient became afebrile), bacteriological efficacy (sterilization of the urine), and biological efficacy (time to normalization of the indices of the acute inflammatory response) were identical for both groups regardless of the duration of iv antibiotic treatment (seven days for amoxycillin/clavulanate; 14 days for cefotaxime). The only side effect was diarrhoea, which affected three patients and did not require modification of the oral treatment regimen.(ABSTRACT TRUNCATED AT 250 WORDS)