We studied 89 men undergoing transrectal prostatic biopsies to determine the efficacy of trimethoprim-sulfamethoxazole in preventing postoperative fever, bacteremia and bacteriuria. Of the patients who received placebo temperatures of 37.6C or more occurred in 18%, bacteremia in 70% and bacteriuria in 21%. Most cases of bacteremia were caused by anaerobic organisms and were asymptomatic. Trimethoprim-sulfamethoxazole did not reduce the frequency of fever or bacteremia but did produce a significant reduction in bacteriuria (0 versus 21%, p equals 0.008).