The ability to predict how easily a stone will break in response to extracorporeal and intracorporeal lithotripsy would be advantageous for the urologist. In a study of fragments from 52 urinary calculi, in vitro microhardness results were found to increase for the series struvite to brushite. Ultrasound velocity measurements performed on 35 intact urinary stones showed a similar trend. This suggests that ultrasound velocity may be a useful indicator of stone strength and one which could be used in vitro to assist the planning of stone treatment.