CaCO3 has been studied in twelve gallstones in which cholesterol predominated. Different polymorphs of CaCO3, as well as carbonate-apatite, were characterised by X-ray diffraction, polarised light microscopy and scanning electron microscopy. Crystals and rhombohedral aggregates of calcite, aggregates of small crystals and tabular crystals of vaterite and fibrous radial aggregates in the form of ooliths were found. In microscopic studies on sections of the stones, CaCO3 was localised mainly in the periphery in the form of layers or bands associated with calcium bilirubinate which was deposited in close relation to it. The phenomena contributing to the precipitation of CaCO3 are discussed, as is the importance of the latter in resistance to the pharmacologic treatment of gallstone disease.