Gallstone disease and cholesterolosis have many common etiologic factors. Gallstone disease is characterized by formation of cholesterol concrements in the gallbladder cavity. Cholesterolosis is a hyperplastic cholecistosis, caused by cholesterol accumulation in the gallbladder wall with subsequent proliferation of gallbladder normal tissue elements [1]. Variations in metabolism of lipids that lead to increases in serum total and low density lipoprotein (LDL) cholesterol, often to bile supersaturation with cholesterol, and possibly gallbladder hypomotility, have been implicated in gallstone and polyp formation [2,3]. The two disorders may be considered as a single disease, caused by impairment of cholesterol metabolism, but exhibiting different forms [4]. Both are clearly multi factorial disorders depending on both genetic and non genetic factors. Family and population studies have revealed a genetic predisposition in about 10-40% of patients with these diseases [5-7]. We describe a pair of monozygotic twins, one of whom developed gallstone disease and the other developed cholesterolosis. This illustrates the common genetic basis of both conditions. Differences between them may be explained by epigenetic, metabolic, or environmental factors.