OBJECTIVE Contemporary treatment of lower pole renal calculi includes extracorporeal shock wave lithotripsy, percutaneous nephrostolithotomy and retrograde ureteropyeloscopy. Success rates for shock wave lithotripsy are reduced in this setting, especially for stones greater than 1 cm. and/or in patients with anatomical variants. Percutaneous treatment, although effective, subjects the patient to increased morbidity. We studied the safety and efficacy of retrograde ureteroscopic treatment of lower pole intrarenal calculi. METHODS We evaluated 90 stone burdens localized to the lower pole and treated with a small diameter, actively deflectable, flexible ureteropyeloscope and a 200 micron holmium laser fiber. Stone burdens were classified as group 1--10 or less, group 2--11 to 20 and group 3--greater than 20 mm. in largest diameter. Patients with calculi less than 2.5 cm. were treated as outpatients unless concurrent medical conditions required hospitalization. Larger stones and partial staghorn calculi (group 3) frequently required 2-stage endoscopic procedures with retrograde intrarenal irrigation for 36 hours to clear debris. An acceptable immediate surgical outcome was defined as complete fragmentation reducing the stone burden to dust and 2 mm. or less fragments. Success was defined as clear imaging (that is stone-free) on renal sonography with minimum 3-month followup. Extreme anatomical variants, including a long infundibulum, acute infundibulopelvic angle and a dilated collecting system, were noted and correlated with surgical failures. RESULTS Endoscopic access and complete stone fragmentation were achieved in 94, 95 and 45% of groups 1, 2 and 3, respectively. After a second treatment the success rate increased to 82% in group 3, with an overall rate of 91%. Of the 19 surgical failures 8 were secondary to inability to access the lower pole and 11 were secondary to inability to render the patient stone-free. In 2 of the 19 cases infundibular strictures hindered ureteroscopic access. In addition, of the anatomical variants a long lower pole infundibulum was the most statistically significant predictor of failure. Mean operative time ranged from 38 minutes for small to 126 for the largest calculi. There were no major complications. Overall stone-free rates with minimum 3-month followup were 82, 71 and 65% in groups 1, 2 and 3, respectively, and 88, 77 and 81%, respectively, in patients with an acceptable initial surgical outcome (that is excluding those with access failures from analysis). CONCLUSIONS Retrograde ureteropyeloscopy is a safe and effective surgical treatment for lower pole intrarenal calculi.