The percutaneous nephrostomy tract has provided urologists with antegrade access to the upper urinary tract. Via this approach ureteropelvic junction obstruction has been treated with an endoscopically controlled incision with a cold knife. While less invasive than an open operation, we have noted significant discomfort from the 24 to 32F percutaneous tract. Accordingly, we sought to achieve the same results by using a less invasive approach, that is retrograde ureteronephroscopic endopyelotomy. A total of 10 patients (9 women and 1 man) with 5 primary and 5 secondary ureteropelvic junction obstructions underwent ureteroscopic endopyelotomy with a 3 or 5F Greenwald cutting electrode passed through a 12F rigid, 10.8F flexible or 9.8F flexible deflectable ureteronephroscope. Preoperatively, ureteropelvic junction obstruction was documented by a furosemide washout renal scan and/or a Whitaker test in 8 of 10 patients. In 2 patients an excretory urogram or retrograde pyelography was diagnostic. Duration of the procedure averaged 180 minutes (245 minutes with a concurent Whitaker test). At the conclusion of the procedure a 7/14F indwelling ureteral stent was placed. The nephrostomy tube was removed after 3 days and average hospital stay was 5 days. The ureteral stents were routinely left in place for 6 weeks. Followup in 10 patients averaged 12 months. Flank pain was largely resolved in all patients. A followup Whitaker test, excretory urogram or renal scan ultimately has demonstrated decreased or no obstruction in 9 or 10 patients. In summary, early results with retrograde ureteronephroscopic endopyelotomy, specifically in female patients, appear to be promising. Morbidity is minimal and efficacy is satisfactory given the favorable objective response noted in 90% of the patients.