The pharmacokinetics and clinical efficacy of amoxycillin combined with clavulanic acid in the treatment of 32 children with urinary tract infection were studied. Twenty one (80%) of 26 children with proved urinary tract infection showed a favourable clinical and bacteriological response. Fifteen of these children had amoxycillin resistant organisms and were treated successfully. In 20 children the serum and urine concentrations of amoxycillin and clavulanic acid were measured after the first oral dose.