Gallstone disease and cholesterolosis may be inde pendent diseases or forms of the same disease, caused by impairment of cholesterol metabolism. 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 gallblad der normal tissue elements. We describe monozygotic twin sisters, one of whom developed gallstone disease and the other developed cholesterolosis. Monozygosity was veri fied by a DNA fingerprinting method. Both had identical heterogeneity of plasma low density lipoprotein (LDL) subfractions, but associated with different functional con ditions within the gallbladder. The different manifestations may be due to epigenetic, metabolic or environmental factors, since both had a common genetic basis.