Cholelithiasis is the primary expression of obesity in the hepatobiliary system. In obese subjects the risk of developing gallstones is increased due to a higher cholesterol saturation of gall-bladder bile. During weight reduction with very low calorie diets (VLCD) the incidence of gallstones increases, but the mechanism for gallstone formation is not completely understood and several pathogenetic mechanisms have been suggested: increased saturation of bile, increased gall-bladder secretion of mucin and calcium, increased presence of prostaglandins and arachidonic acid. Alterations in gall-bladder motility may contribute to gallstone formation, but few studies have addressed the issue of gall-bladder motility during rapid weight loss and its possible role in gallstone formation. VLCD have been associated with a gall-bladder stasis, as a consequence of reduced gall-bladder stimulation by low fat content of the diets. A threshold quantity of fat (10 g) has been documented to obtain efficient gall-bladder emptying. Ursodeoxycholic acid administered during VLCD seems to have a protective role in developing a biliary cholesterol crystals. Gall-bladder emptying was lower in response to low fat meals with respect to relative higher fat meals, before as well as during the VLCD. This may account the possibility of an adaptative response of the gall-bladder motility to a given diet regimen. Adequate fat content of the VLCD may prevent gallstone formation, maintaining adequate gall-bladder motility and may be more economic and physiologically acceptable than administration of a pharmacological agent.