Three hundred and forty-three patients with suspected bacteriuria undergoing transurethral resection of the prostate (TUR-P) were randomized to treatment with either trimethoprim-sulfamethoxazole (TMP-SMX) or norfloxacin (NF) for 5 1/2 days beginning the evening prior to operation. It was possible to analyse 165 patients for efficacy. Elimination of bacteria on days 10 to 20 was achieved in 78.1% and 78.3% in the TMP-SMX and NF group, respectively. The accumulated elimination rates for the follow up period (days 10-42) were 68.5% for the TMP-SMX group and 76.2% for the NF group. The differences were not statistically significant. No patient had any clinical signs of upper urinary tract infection or septicemia. Three hundred and twelve patients were analysed for safety. Twenty-six patients reported 32 adverse drug events (ADEs). Four reactions in the TMP-SMX group were considered severe while in the NF group all the ADEs were of mild or moderate intensity. In this study NF seems to be at least as effective and safe as TMP-SMX.