Purpose: To assess the incidence of conversion from laparoscopic partial nephrectomy (LPN) to open surgery or laparoscopic radical nephrectomy (LRN) when liberal selection criteria are utilized. Patients and Methods: A retrospective review of medical records was done for all patients scheduled for LPN at our institution from January 2000 through March 2004. The preoperative risk factors, intraoperative course, and pathologic outcomes of patients who were converted to LRN were compared with those of the cohort of patients who underwent LPN as originally scheduled. Results: Among the 257 operations that started as LPN, 35 (13.6%) were converted to LRN and 4 (1.6%) to open surgery. Age, tumor size, operating time, and hypertension were significantly higher in patients requiring conversion in than those who underwent completed LPN. Patients over the age of 70 had a 3.8-fold higher risk of requiring conversion, and, independent of age, patients with tumor >4.0 cm had a 4-fold increase in the likelihood of conversion to LRN. Conclusion: Of the preoperatively determined factors compared across the cohort of patients who underwent LPN and the cohort of patients converted to LRN, only tumor size and patient age were predictive of an increased risk of conversion. Other variables, including sex, side of affected kidney, clinical stage, ASA score, comorbidity with hypertension or diabetes mellitus, and surgeon were not significant in predicting conversion. © Mary Ann Liebert, Inc.