Transmission electron microscopic studies have been carried out on psammoma bodies in two benign and seven malignant papillary serous neoplasms of the ovary. Ultrastructurally, psammoma bodies are composed of microcrystals in all respects similar to calcium-phosphate apatite crystals of bone. The formation of psammoma bodies is initiated intracellularly, in both the neoplastic epithelial cells and stromal histiocytes. The initial seeding site of apatite crystals is served by lipid-rich intracellular vesicles. These structures are produced in association with autophagocytosis in the neoplastic epithelial cells and heterophagocytosis of extracellular lipidic material in the stromal histiocytes. Extracellular lipids presumably derive from dehiscent tumor tissue. The close relationship between larger intraepithelial calcific bodies and microfilaments suggests that the latter provide supportive matrix for further intracellular calcification. Large extracellular psammoma bodies result from fused calcific bodies which have been extruded from calcified cells. Mineralization of extracellular collagen fibers is not observed. The results provide supportive evidence to the concept that psammoma bodies in ovarian papillary serous neoplasms and probably in other neoplastic and non-neoplastic conditions are a consequence of dystrophic calcification associated with cellular degeneration.