Forty-two men with urinary tract infection and benign prostatic hyperplasia were randomized into two groups before transurethral resection. One group (22 patients) received Claforan (cefotaxime) peroperatively and thereafter daily for five days. In the other group (20 patients), Hiprex (methenamine hippurate) was given daily from the day before the operation, for a total of six days. All the bacterial isolates were sensitive to cefotaxime. The efficacy of the medication was clinically and bacteriologically evaluated. Postoperative temperature elevation (greater than 38 degrees C) occurred in one of the 22 patients in the cefotaxime group, and in nine of the 20 in the methenamine hippurate group (p less than 0.05). None of the former group, but two patients in the latter, had septicemia. The difference was not statistically significant. The response to treatment was satisfactory in 13 of the 22 patients in the cefotaxime group, but in only one of the 20 treated with methenamine hippurate (p less than 0.005). Antibiotic treatment is recommended for bacteriuric patients undergoing transurethral prostatic resection.