137 patients were treated by ESWL with Sonolith 2000 lithotriptor using ultrasonography localisation. 132 patients were eligible for review. Calculi were caliceal in 85, pelvic in 51, in upper ureter in 1; 26 stones were treated by the push and bang technique with an ureteric stent. Fragmentation rate was 83.9% and stone free rate at 3 months was 68.5%. Failure were reviewed: non fragmentation (16.1%) and insufficient fragmentation (15.1%). 14 calculi were analysed by infra red spectrophotometry and correlated with radiodensity on pre ESWL plain X-ray. Hardness and friability criteria are described. Calculi of different radiographic aspects respond differently to shock wave fragmentation. Some stone composition as calcium oxalate monohydrate pure or mixed with apatite and calcified uric acid are difficult to break by ESWL. Those calculi represented almost 30% of the series. This criteria of hardness has to be taken into account for the selection of patients in order to improve the results of ESWL.