Pathomorphological investigations of the cholesterol crystals were carried out in the middle ear effusions and cholesterol granulomas of experimented animals and humans. The shape and size of the cholesterol crystals were variable, depending upon the surrounding substances. In the middle ear effusions, the crystals formed rather large rectangular structures, which tended to become smaller and thinner, as the granulomas developed. In the compact granulomas, the crystals appeared smaller, thinner and deformed, suggesting that they may possible be dissolved by a phagocytosis of active giant cells.