We evaluated the respective role of size, location and composition of renal stones as determinants of the outcome of extracorporeal shock wave lithotripsy (ESWL). The analysis of the chemical composition and of the morphology of the fragments eliminated by hundred patients who underwent ESWL has been made by X-rays diffraction. Results showed that the larger the stone, the higher the risk of having to perform an extra intervention (additional sessions of ESWL, percutaneous nephrostolithotomy, nephrolitholapaxy) or of finding residual fragments at the three month check-point. In addition, the location of the stone in a calice, especially a lower calice, strongly predisposes to incomplete elimination of the fragments. Finally, two types of stones appear to lessen the probability of success of ESWL: those of struvite and those of cystine. Indeed, out of eleven ESWL performed on struvite stones (pure or mixed), two had to be completed by percutaneous nephrolitholapaxy and even one by a nephrectomy because of superinfection. Out of the four patients with cystine stones, two had to be treated by additional percutaneous nephrolitholapaxy because of inadequate fragmentation after ESWL. However, for the remaining types of stones (pure or mixed) chemical composition seems to be less important than size or location.