Objectives: To evaluate the factors that affect the success rate of extracorporeal shock wave lithotripsy (ESWL) for treatment of renal stones. Patients and methods: Between January 2000 and December 2003, 427 patients with single or multiple renal stones (<30 mm, largest diameter) underwent ESWL monotherapy using Storz SL 20 lithotriptor. The results of treatment were evaluated after 3 months of follow-up. Treatment success was defined as complete clearance of the stones or presence of clinically insignificant residual fragments <4 mm. The success rate was correlated with the characteristics of the patients, conditions of the urinary tract and stone features. Results: At 3-month follow-up, the overall success rate was 333/427 (78%). Repeat treatment was needed in 226 patients (53.1%). Post-ESWL auxiliary procedures were required in 36 patients (8.4%). Post-ESWL complications were recorded in 16 patients (3.7%). Of the 10 prognostic factors studied, 5 had a significant impact on the success rate, namely: renal morphology, congenital anomalies, stone size, stone site and number of treated stones. Other factors including age, sex, nationality, stone nature (de novo or recurrent) and ureteric stenting had no significant impact on the success rate. Conclusions: The success rate of ESWL for the treatment of renal stones could be predicted by stone size, location and number, radiological renal features and congenital renal anomalies.