Prospective, randomized study in 96 patients undergoing urologic endoscopic surgery to assess the efficacy of two antimicrobials, pefloxacin and ceftriaxone, in the prevention of urinary tract infections. The disease most frequently prompting surgery was vesical tumour and prostate adenoma. All patients had sterile pre-operative urine. Drugs were given endovenously two hours prior surgery in one single dose. Prophylactic efficacy was measured through urine culture before removal of the catheters. Incidence of bacteriuria greater than 100,000 colonies/ml was 20% for the pefloxacin group compared to 15.2% for ceftriaxone. These differences were not statistically significant (Squared-chi test). These results show that pefloxacin is as effective as ceftriaxone in the prophylaxis of endoscopic urological surgery. A single dose schedule was not entirely suitable to reduce post-operative bacteriuria.