The fine structure and pathogenesis of asteroid hyalosis (AH) were investigated in this study. Samples were taken at vitreous surgery and trapped on membrane filters. Particles in the samples were observed with light and electron microscopes and processed by the focused ion-beam method. Chemical analysis was performed with an energy-dispersive X-ray microanalyzer. In vitro, reconstructive studies of hydroxylapatite were performed in simulated body fluid (SBF). AH is about 180 microm in diameter, spheric, and covered with a transparent vitreous matrix; its naked surface resembles reconstructed particles of hydroxylapatite (HOA). The asteroid bodies (AB) appeared as snowballs under the scanning electron microscope; the surface of the ABs was covered with clotted matrix of vitreous substances, and the structure under the covered substances showed a pumice stonelike structure. Artificial HOA produced by the biomimetic method built a surface pattern similar to the AH particles on a collagen-coated styrene plate. We concluded that AH was produced not only by changing of ionic tension in the vitreous fluid but also by changes of vitreous matrix in the aging process and in diseases. The AH is composed of HOA, which is produced by the body, and its chemical constitution is phosphate and calcium. These structural characters resemble stones in the living body (lithiasis); thus, we concluded that AH is a condition of lithiasis in the body.