Urolithiasis is one of the oldest diseases and continues to be a major problem in India. PCNL continues to be the standard of care in selected cases according to the stone size, location, shape and composition of the stone. PCNL is recommended for cases with stones larger than 20mm, cases with struvite or cysteine stones, or in cases of anatomical variation. The outcomes of PCNL is interpreted in terms of success of the procedure and complication rates. Multiple factors have been investigated as predictors of success rates and complications. A quick, simple and reproducible method for the prediction of the outcomes of PCNL was proposed by Thomas et al called as the ‘Guy’s stone score’. The grading system mainly takes into consideration the number of stones, stone location and whether the renal anatomy is simple or abnormal. Currently there is no single agreement upon an ideal predictive model that characterizes the complexity of renal stones predicting surgical outcomes following PCNL. Hence the present study was conducted for evaluating the role of GUY’s score in the outcome of PCNL. Materials and methods: Patients scheduled to undergo PCNL at Ramaiah Hospitals were assessed for eligibility for the study and those satisfying the inclusion criteria were included in the study. The stone burden was determined by radiographic studies, and stones will be classified using the GSS as Guy's I, II, III and IV. Post-operative stone clearance rate was assessed by any residual shadow in x-ray KUB in the immediate post-operative period and NCCT after 3 months and patients were followed up to note any complications in the postoperative period. And stone free status was assessed Results and discussion: 200 patients scheduled to undergo PCNL were included. Incidence of complications was more in patients with GUY’S grade IV (P<0.01). Incidence of relook PCNL was more in patients with GUY’S grade IV. stone free clearance was more in patients with GUY’S grade I and II, stone size <200mm2, with essence HU <800 HU, with lower calyx puncture. We have also observed that incidence of complications pleural effusion, urine leak and bleeding was more with Guy’s IV and more with superior calyx puncture. Conclusion: We conclude that GUY’S score is a useful tool in the initial evaluation to predict surgical outcomes in PCNL and can help in better pre-operative planning of the surgery. Keywords: Guy's score; PCNL; Urolithiasis; Outcome; Prediction.