The efficacy of a long-term prophylaxis with Co-trimoxazole was studied in 18 girls with recurrent urinary tract infections. SEven patients had neither anatomical nor functional anomalies of the urinary tract. Eight patients had a vesico-ureteral reflux, and three patients a complex urinary problem. The following therapeutic plan was applied: administration of a low dose of Co-trimoxazole (TMP: 0,6-1 mg/kg) given as a single evening dose during 3 months followed by a 3-month period without treatment. The children were followed in this way during 2 years. Seven children had no reinfection from the start of the prophylaxis during the 2 years of observation. Nine children had a significant decrease of the number of reinfections during the Co-trimoxazole treatment periods. Two children without anatomical anomaly had early reinfections with Co-trimoxazole resistant germs. No side effects related to Co-trimoxazole have been observed, and the tolerance of treatment was excellent. Co-trimoxazole administered as a single evening low dose efficiently prevented the recurrence of urinary tract infections.