Using light and scanning electron microscopy thin epoxide processed sections of 12 impacted third molars, one molar developed in odontogenic follicular cyst and one tooth-like teratoma were studied. It was established that the 4 impacted teeth and the tooth developed in odontogenic follicular cyst had obvious signs of enamel damage in the form of erosion in the crown fissures. The features of enamel alteration make it possible to interpret the changes as carious process which apparently has endogenous origin. Tooth-like teratoma extracted from ovarian dermoid cyst looked like canine and incisor "hybrid" coronal portion of which is covered with thin enamel layer, marked by obvious signs of multiple erosive lesions of endogenous nature. But unlike alterations in impacted teeth it resembles either fluorosis or enamel hypoplasia.