OBJECTIVE The percent of functional volume preservation is a primary determinant of functional outcome after partial nephrectomy. We assessed what is to our knowledge a novel method to estimate the percent of functional volume preservation to assess its effect on functional outcomes. METHODS We studied the glomerular filtration rate outcome based on the modification of diet in renal disease 2 in 39 patients with normal preoperative serum creatinine who underwent open or laparoscopic partial nephrectomy from January 2007 to December 2009. A cylindrical volume ratio method was used to estimate the percent of functional volume preservation on computerized tomography images obtained before and after partial nephrectomy. A model to predict the postoperative estimated glomerular filtration rate was based on multiplying the preoperative glomerular filtration rate by the percent of functional volume preservation, followed by adjustment for the functional contribution of the contralateral kidney. Correlation and multiple regression analysis was done to test the model. RESULTS The median preoperative, nadir and late estimated glomerular filtration rate in the cohort was 104 (range 53 to 234), 75 (range 21 to 189) and 90 ml per minute/1.73 m2 (range 45 to 228), respectively. The nadir and late estimated glomerular filtration rate was measured at a median of 2 (range 0 to 8) and 358 days (range 13 to 827), respectively. The median percent of functional volume preservation was 88% (range 50% to 100%) for the operated kidney and 94% (range 75% to 105%) when adjusted for total bilateral kidney volume. We noted a 96% correlation between the predicted and the observed late estimated glomerular filtration rate. On multivariate analysis the preoperative glomerular filtration rate (p<0.001) and ischemia time (p=0.02) correlated with the nadir glomerular filtration rate, and the preoperative glomerular filtration rate (p<0.001) and the percent of functional volume preservation (p=0.04) correlated with the late glomerular filtration rate. CONCLUSIONS These data support the notion that preoperative nephron endowment and the percent of functional volume preservation are the primary determinants of the long-term functional outcome after partial nephrectomy in patients with normal preoperative kidney function who have ischemia time within acceptable limits.