Eighty-one patients with proved preoperative sterile urine and undergoing transurethral resection of the prostate were studied. The patients were divided into 3 groups: group A received sulfamethoxazole-trimethoprim (ST) preoperatively and postoperatively for ten days; group B received ST in 2 divided doses, one pre- and one postoperatively; group C received no prophylaxis. In groups A and B, we found urinary infection in 3.8 per cent of patients compared with 32 per cent in group C. Performing prostatic chip cultures, we found that most urinary infections were unrelated to a prostatic source. When the prostate was infected, 75 per cent had infected urine postoperatively. We believe that prophylactic antimicrobial treatment should be given to all patients undergoing transurethral prostatectomy. However, it seems that immediate perooperative treatment suffices.