Two distinct forms of microcalcification are found in breast disease. The more commonly recognized type is basophilic and nonbirefringent and consists predominantly of calcium phosphates (type II). The other type is a birefringent, colorless crystal that is composed of calcium oxalate (type I). It has not been emphasized in the literature that calcium oxalate can produce radiopacities and yet is easily overlooked in tissue sections. In this study, histologic sections taken from 127 randomly selected, mammographically directed biopsies were reviewed and the presence or absence of microcalcifications, the type of microcalcification, and the associated disease process were noted in each case. Seventy-eight of the 127 specimens were obtained because of suspicious calcifications detected by mammography and 9 (11.5%) contained only type I microcalifications, 9 (11.5%) contained both types I and II microcalcifications, and 48 (61.5%) contained only type II microcalcifications; in 12 specimens (15.4%), microcalcifications were not identified. The overall incidence of type I calcifications was 17.3% (22/127), but the incidence in those specimens obtained because of calcifications detected by mammography was 23.1% (18/78). Type I microcalcifications were found only in benign cysts and were not associated with carcinoma or epithelial hyperplasia, whereas type II microcalcifications were associated with benign or malignant lesions. These findings are in keeping with the hypothesis that type I microcalcifications are a product of secretion whereas type II microcalcifications are a result of cellular degeneration or necrosis. In biopsies in which type II microcalcifications are not identified, examination of sections under polarized light may reveal the presence of calcium oxalate crystals.