To investigate the pathophysiological basis of Courvoisier's law, the gallbladder size and pliability were measured in chronic calculous cholecystitis with (7 patients) or without (25 patients) common duct obstruction, in periampullary malignancy (6 patients), and in the normal state (3 patients). The duration and level of jaundice, when present, were correlated to the gallbladder size and ductal pressure as measured at operation. It was found that in vitro the gallbladders were equally pliable in all groups. Therefore, the classical explanation, that stone obstruction of the common duct does not result in dilated gallbladders because of less pliable organs from preexisting fibrosis, does not seem to apply for this series of patients. A markedly higher ductal pressure and a longer history of and deeper jaundice were also found in patients with dilated gallbladders. The data of this study support the alternative explanation that chronically increased ductal pressure is the probable cause of dilated gallbladders seen in malignant obstruction of the common duct.