Ultrastructural studies were performed on five cases of asteroid hyalosis of the human vitreous. The large asteroid bodies were composed if intertwined ribbons of multilaminar membranes with a 6-nm periodicity characteristic of complex lipids, especially phospholipids, lying in a homogeneous background matrix. Clumps of dense material were interspersed throughout the asteroids, and swirls of thin membranous strands often surrounded them. The structure of small asteroid bodies suggested that the lipid ribbons resulted from deposition of successive lipid layers on the thin membranes, so that bodies with variable degrees of lipidization occur. By energy-dispersive x-ray analysis, calcium and phosphorus were the main elements detectable in asteroid bodies of all sizes. Electron-diffraction structural analysis showed that calcium hydroxyapatite and, possibly, other forms of calcium phosphate crystals were present in the focal densities. Calcium may also be bound to phosphate groups in the phospholipids. The large quantity of complex lipids and calcium in asteroids bodies suggests a derivation from sources exogenous to the vitreous.