Long-term low-dosage prophylaxis is used in childhood urinary tract infection to prevent recurrence after the initial infection has been eradicated. Its use is indicated (1) in children whose kidneys are liable to damage from repeated infections, that is, those with vesico-ureteric reflux and chronic pyelonephritis (children with obstruction of the lower urinary tract having been identified and treated surgically) and (2) in children even with normal urinary tracts who are troubled by repeated symptomatic infections. (Normand & Smellie, 1965). For this purpose a drug is required which is effective, well tolerated, free from side effects and does not induce drug resistance. We have shown nitrofurantoin to be an efficient prophylactic drug (Gruneberg et al, 1973) but as it occasionally causes nausea we have also studied co-trimoxazole.