Percutaneous extraction of renal stones is associated with a risk of infection, which sometimes can be severe as a result of the intraoperative introduction of a ureteral catheter, the nephroscopy itself and the fact that a nephrostomy tube sometimes is left in place. It generally is accepted that patients with a preoperative urinary tract infection should be covered during the operation by an appropriate antibiotic. However, the need for routine prophylactic antibiotic treatment in patients with sterile urine preoperatively still is a subject of debate. We report the bacteriological results of 126 cases of percutaneous extraction of renal stones. Of the patients 107 had sterile urine preoperatively and deliberately did not receive prophylactic antibiotics so that the mechanisms of urinary tract infection after percutaneous nephrolithotomy could be studied. Of these patients 37 (35 per cent) suffered a postoperative urinary tract infection, usually owing to Escherichia coli, streptococcus or staphylococcus. The responsible organism was isolated in the bladder urine only in 22 cases, in the nephrostomy tube in 2 and in both sites in 13. Eleven patients (10 per cent) presented with a fever of 38.5C or more. All of the infected patients received appropriate antibiotic therapy and there were only 2 bacteriological failures on long-term followup (5 per cent). A total of 19 patients had a urinary tract infection preoperatively. All 19 patients received appropriate antibiotic therapy starting at least 24 hours preoperatively and continuing for a minimum of 3 weeks. Five patients (26 per cent) presented with a fewer but there were no serious septic complications. All of the patients were discharged from the hospital with sterile urine and there was only 1 long-term bacteriological failure (5 per cent). Both patients with Pseudomonas infection were cured. The risk of clinical infection following percutaneous nephrolithotomy is low despite the fact that 35 per cent of the patients have bacteriuria postoperatively, provided a careful bacteriological examination is performed preoperatively and the patients with urinary tract infection are treated appropriately. These results are in favor of short-term prophylactic antibiotics adapted to the bacterial ecology.