Using transmission electron microscopy, the fine structure of anterior persistent hyperplastic primary vitreous (PHPV) removed from the eyes of four infants was studied. The tissue mass was composed of vessels derived from or representing the tunica vasculosa lentis posterior and the vasa hyaloidea propria. They were embedded in a loose connective-tissue matrix containing many fibroblasts and mature collagen fibrils. Toward the posterior surface, the fibroblasts became more numerous, elongated and densely packed. At the point of its entry into the posterior pole of the mass, the hyaloid artery was found to be surrounded by glial cells, probably representing an extension of the wall cells of the hyaloid canal. Venous drainage of the PHPV nodules seemed to occur via the ciliary body to which they were connected by tiny vessels bridging the distance from the PHPV lump to elongated ciliary processes. The anterior surface of the tissue lump was covered with lens fragments. These remnants showed signs of very early disturbance of lens development, with a failure to form posterior lens fibers. Based on these findings, the authors suggest that PHPV may be secondary to a primary defect in lens development.