To evaluate the fragility of Ca-oxalate monohydrate stones in terms of high energy shock wave effectivity, we measured the glycosaminoglycan (GAG) content of the organic stone matrix of 35 patients who had undergone extra-corporeal shock wave lithotripsy (ESWL) treatment for renal pelvic stones. The patients were classified into 3 different groups with respect to the rate of stone disintegration. While group I consisted of patients in whom only one ESWL session was needed to complete disintegration, in group II there were patients requiring 3 or more sessions for complete stone disintegration. On the other hand, patients in whom no sign of disintegration was observed despite 3 ESWL sessions constituted group III. Following this procedure, comparative evaluation of the GAG content of the stone matrix per dry stone weight in each group revealed a statistically significant difference between the first 2 and the 3rd group (p < 0.05). Our results indicated that as the GAG content of the stone matrix decreased, the efficiency of the shock waves on stone disintegration also prominently decreased, disintegration being impossible in some cases. On the other hand, the stones which were found to have higher amounts of GAGs in the organic matrix tended to be more fragile and were easily disintegrated with shock waves. In our study group the chemical composition of Ca-oxalate monohydrate stones, e.g. the matrix GAG content, seemed to be important with regard to the disintegrative effect of high energy shock waves.