The effectiveness and drawbacks of cefoperazone and cotrimoxazole in the prevention of postoperative infections following percutaneous removal of renal stones were studied comparatively. 60 patients were divided at random into two groups. 30 subjects were given 1 g cefoperazone IV every 8 hours for 5 consecutive days starting on the day before the procedure. The 30 other patients had an infusion of 800 mg sulfamethoxazole and 160 mg trimethoprim every 12 hours on the same 5 days. Age, sex and type of surgical procedure were comparable in both groups. Results were as follows: in the cefoperazone group, one patient had intraoperative septic shock due to a stone infected by a resistant Pseudomonas aeruginosa; in the cotrimoxazole group, 2 patients had postoperative fever due to stones infected by resistant Gram negative rods (Pseudomonas aeruginosa) and three patients had a urinary tract infection (Candida albicans in 1 case, Escherichia coli in 1 and Pseudomonas aeruginosa in 1). Tolerance was satisfactory for both regimens. The authors conclude that intravenous cefoperazone in the more effective drug and should be continued throughout the first three postoperative days.