The technical results of 259 nephron sparing operations for renal cell carcinoma or renal oncocytoma were reviewed. Local or renal related complications occurred after 78 procedures (30.1%). The incidence of complications was less for operations performed after 1988 (22% versus 37%, p = 0.009) and for incidentally detected versus suspected tumors (p = 0.009). The most common complications were urinary fistula formation (45 operations) and acute renal failure (33). Significant predisposing factors for urinary fistula formation included central tumor location (p = 0.001), tumor size greater than 4 cm. (p = 0.001), the need for major reconstruction of the collecting system (p = 0.001) and ex vivo surgery (p = 0.001). Only 1 urinary fistula required open operative repair, while the remainder resolved either spontaneously (30) or with endoscopic management (14). Significant predisposing factors for acute renal failure included a solitary kidney (p = 0.001), tumor size greater than 7 cm. (p = 0.008), greater than 50% parenchymal excision (p = 0.001), greater than 60 minutes of ischemia time (p = 0.035) and ex vivo surgery (p = 0.001). Acute renal failure resolved in 28 patients, of whom 9 required temporary dialysis, while 5 required permanent dialysis. Overall, 8 complications (3.1%) required repeat open surgery for treatment while all other complications resolved with noninterventive or endourological management. Surgical complications contributed to an adverse clinical outcome in only 7 patients (2.9%). Nephron sparing surgery can be performed safely with preservation of renal function in most patients with renal tumors.