Extracorporeal shock-wave lithotripsy (ESWL) is currently being evaluated as a treatment for symptomatic gallstones in the USA. It was first used to treat kidney stones in humans approximately 10 years ago, and now 85% of all kidney stones can be treated with this method [1]. Although ESWL can efficiently fragment gallstones, rendering them more amenable to dissolution therapy with oral bile acid therapy [2], methyl-ten-butyl ether (MTBE) [3] or Monooctanoin [4], at present it is successfully used only in a select group of patients. Nevertheless, shock-wave therapy remains a viable treatment for gallstones, particularly for those patients who prefer non-surgical treatment or for those who are physically unfit for surgery. Cholesterol is the major component of most gallstones found in the USA and Europe, the remaining components being calcium bilirubinate, calcium carbonate and mixed bile pigments. Only patients who are suspected to have cholesterol gallstones (defined as 70% cholesterol by weight) [5] are being treated with ESWL in order that the remaining fragments be dissolved. It is estimated that 10-25% of all people suffering from cholelithiasis (gallstone disease) can be successfully treated by ESWL [6]. With this in mind, some lithotripter manufacturers are attempting to develop machines that may treat both kidney stones and gallstones. The mechanical properties of these stones, however, are very different. This work was aimed at determining the fracture strength of human gallstones and investigating the effect of cholesterol content on this fracture strength. There are very few published data on the mechanical properties of gallstones. In one study two gallstones were tested by diametral compression [7]. These stones had fracture strengths of 0.23 MPa (33 lb in.-2) and 0.31 MPa (44 lb in.-2). In comparison, similar tests performed on kidney stones yielded fracture strengths ranging from 0.45 to 1.54MPa (65 to 2241bin. -2) [8]. In testing both kidney stones and gallstones, completely brittle behaviour of the stones was observed. These stones had been dried and rehydrated before testing. The present work serves to extend the determination of fracture strength of gallstones by diametral compression testing and to determine any correlation of cholesterol content with this fracture strength. Gallstones cannot readily be formed into standard tensile test samples for use with normal tensile testing machines. However, one test, the diametral compression test (sometimes called the Brazilian