473 Background: The word heminephrectomy originally referred to the removal of half of a kidney with a duplicated collecting system. In the era of partial nephrectomies, heminephrectomy is sometimes used to describe the removal for tumor of >30% of the parenchyma of a non-duplicated kidney. We herein propose a new definition of heminephrectomy: excision of the upper or lower pole of the kidney, removing at least 30% of the parenchymal mass, cutting to hilar fat, and transecting the collecting system. Methods: Our institutional database of robotic and laparoscopic partial nephrectomies was queried for patients who underwent a partial nephrectomy for tumor between 2002 and 2011. Patients who had a heminephrectomy for obstruction in a duplicated collecting system were excluded. The patients who had a heminephrectomy by our strict definition were compared to the remainder of patients who had a partial nephrectomy. Logistic regression was used to compare outcomes between the two groups. Results: 61 patients met our strict definition of heminephrectomy out of 643 patients who underwent a partial nephrectomy for tumor. Heminephrectomy and non-heminephrectomy patients were similar in age, gender, BMI, ASA score, proportion of left- versus right-sided tumors, solitary kidney status, preoperative creatinine and GFR. (all p>0.1). The tumors in the heminephrectomy group were larger (5.1 vs. 2.8 cm, p<0.001) and had a higher R.E.N.A.L nephrometry score (8.5 vs. 6.5, p<0.001). Operative outcomes reflected the high complexity of performing a heminephrectomy. Estimated blood loss was greater (373 vs. 267 ml, p=0.04), operative time was longer (214 vs. 185 minutes, p<0.001), warm ischemia time was longer (25 vs. 20 minutes, p=0.002), and the rate of intra-operative complications was greater (11% vs. 4%, p=0.02) in the heminephrectomy group. On multivariable analysis adjusted for age, preoperative GFR, R.E.N.A.L. score, and warm ischemia time, heminephrectomy was a significant predictor of lower post-operative GFR (p<0.001). Conclusions: Our strict definition of heminephrectomy performed for tumor in a non-duplicated system will allow precise clinical and research communication about heminephrectomy patients and may aid in the prediction of outcomes after partial nephrectomy.