Objective To evaluate whether excised parenchymal mass (EPM) during partial nephrectomy (PN) correlates with functional decline and can serve as a surrogate for functional outcomes. Materials and Methods All 215 patients managed with PN for unifocal renal mass with necessary studies to determine EPM and percent glomerular filtration rate (GFR) and parenchymal mass preserved (both global and specific to the operated kidney) were analyzed. EPM was estimated from the pathologic specimen by subtracting the tumor mass from the specimen mass, with both calculated using the elliptical formula. Vascularized parenchymal mass preserved was measured from computed tomography scans obtained <2 months prior and 3-12 months after surgery. All functional analyses were required to be within the same time frames, and patients with a contralateral kidney were also required to have nuclear renal scans. Results The median tumor size was 3.5 cm and the median R.E.N.A.L. was 7. Warm and cold ischemia were utilized in 123 and 92 patients, respectively (median ischemia time = 23 minutes). The median global GFR preserved was 89%, the median total parenchymal mass preserved was 93%, and the median estimated EPM was 16 cm3. Whereas percent parenchymal mass preserved correlated strongly with global and ipsilateral GFR preserved (both P < .001), EPM failed to correlate with functional outcomes on both univariable and multivariable analyses. Conclusion Our data suggest that parenchymal mass preserved with standard PN by experienced surgeons associates strongly with function preserved, whereas EPM fails to correlate with functional outcomes. Further study of the functional impact of EPM in other circumstances will be required, such as enucleation or PN performed by less-experienced surgeons.