INTRODUCTION AND OBJECTIVES: Both Guy’s (GSS) and S.T.O.N.E. Nephrolithometry Scoring Systems have been recently validated as predictors of stone-free status post-Percutaneous Nephrolithotomy (PCNL). Therefore, the aim of the present study was to compare the accuracy of the GSS and S.T.O.N.E. scoring systems in predicting PCNL outcomes. METHODS: After obtaining ethics approval, medical records of patients undergoing PCNL between 2009 and 2013 at a tertiary stone referral centre were retrospectively reviewed. GSS and S.T.O.N.E. Nephrolithometry scores were calculated. Logestic regression analysis and ROC curves were generated to compare accuracy of each scoring system. RESULTS: A total of 185 PCNLs were reviewed. Mean patient age was 55.2±14 years with mean body mass index of 27.8±6.2 kg/m2 and mean operative time of 98.7±34.2 minutes. The overall stone-free rate was 71.9% with complication rate of 16.2%. When compared with patients with residual fragments, stone-free patients had significantly lower Guy’s Score (2.7 vs 2; p p