OBJECTIVE: Prostate biopsy is a frequent diagnostic measure to detect prostatic conditions, including primarily prostate cancer. Its performance does not follow a pattern, mainly regarding preparation for the examination through antimicrobial prophylaxis. The aim of this study is to compare the efficiency of short and long term antimicrobial prophylaxis in transrectal ultrasound-guided prostate biopsies. MATERIALS AND METHODS: A clinical study was conducted with a total of 140 patients submitted to prostate biopsy with transrectal ultrasonographic control. Patients were randomly separated in two groups: Group 1 receiving norfloxacin 400mg single dose before the procedure and Group 2 receiving norfloxacin 400mg initiating before the procedure and then bid up to 6 doses. Efficiency control was determined by the incidence of urinary tract infection (UTI) and complications in both groups after statistical analysis. RESULTS: The incidence of minor complications in our study was 75%, consistent with data found in the literature. Among the patients with diagnosis of simple UTI, 23% belonged to Group 1 and 8% to Group 2 (p=0.08). Among the patients with complicated UTI, 37% belonged to Group 1 and none belonged to Group 2 (p=0.44). CONCLUSIONS: Several advances made transrectal ultrasound-guided prostate biopsies a useful and safe diagnostic tool in the workup of urologic patients. There is no optimal prophylactic preparation to the performance of this procedure. Long term antimicrobial prophylaxis presents a trend toward lower incidence of infectious complications.