Objectives. To evaluate the results of performing percutaneous nephrostomy (PCN) in all patients with kidneys with ureteropelvic junction obstruction (UPJO) and split renal function (SRF) of less than 10%, because the management of such cases is still under debate. If SRF improves, we perform pyeloplasty, otherwise nephrectomy is done. Methods. The records of all patients with UPJO (n = 20; 17 unilateral and 3 solitary kidney) with poor function who underwent PCN were analyzed. The PCN remained in situ for at least 4 weeks, during which patients received low-dose oral trimethoprim prophylaxis. Repeated renography was obtained after 4 weeks. If no improvement in the SRF had occurred, nephrectomy was performed, otherwise pyeloplasty was performed. The patients were followed up with renograms at 3 months, 1 year, 2 years, and 5 years. Results. Twelve of 17 kidneys with unilateral UPJO improved after PCN drainage and underwent pyeloplasty. The 5 kidneys that did not show improvement in the SRF underwent nephrectomy. In the patients with unilateral UPJO who improved after PCN drainage, the SRF increased to 29.2% +/- 12.6% and pyeloplasty was performed. At a mean follow-up of 2.3 years, none of these patients had developed hypertension, and the most recent SRF value was 31.4% +/- 12.8%. Conclusions. Most of the poorly functioning UPJO kidneys show improvement in function and not all such kidneys should be removed without a trial of PCN. UROLOGY 57: 547-550, 2001. (C) 2001, Elsevier Science Inc.