BACKGROUND AND PURPOSE: The R.E.N.A.L nephrometry score (NS) was developed to characterize renal tumor anatomy to facilitate standardized reporting and ultimately clinical decision making. Up to three points are assigned for each of the following criteria: Tumor size (R), exophytic vs endophytic nature (E), nearness to the collecting system (N), anterior vs posterior (A), and polar location (L), with more complex lesions receiving higher scores. There are no independent studies to date that validate the reproducibility of this scoring system. Our aim was to validate the R.E.N.A.L. NS system by assessing interobserver variability, and therefore reproducibility and fidelity of this proposed assessment tool. PATIENTS AND METHODS: We reviewed our prospectively collected laparoscopic partial nephrectomy (LPN) database and identified 306 patients with available preoperative CT or MRI. Of these, 149 were independently read by two urology residents who assigned NS. The Pearson test was used to assess interobserver variability of total NS as well as each of the five components of the scoring system. RESULTS: Interobserver correlation of total NS calculated by the Pearson test was found to be 0.92 (P<0.001). Concordance rates for each of the individual nephrometry components R.E.N.A.L (hilar) were 96%, 92%, 86%, 96%, 89%, and 99% respectively. A t test showed no significant difference between final NS assigned by two different observers. CONCLUSION: The R.E.N.A.L. NS system is a comprehensive and reproducible tool that may aid surgeons in communicating tumor characteristics effectively. Interobserver correlation is high, rendering it a high fidelity assessment tool.