The relation between extension of extraretinal vasoproliferation and vitreous fibers in stage 3 mild retinopathy of promaturity was evaluated by light and electron microscopy using 4 eyes obtained after autopsies. Seven developing human eyes from 20 to 40 weeks' gestation were used as controls. At an early stage, when vascular tissues broke the inner limiting membrane of the retina and grew into the vitreous cavity, they extended vertically from the retina. Proliferating extraretinal vessels extended towards the posterior surface of the lens. They ran side by side with the framework of vitreous fibers. Electron microscopy demonstrated multiple attachments between vascular endothelia and vitreous fibers. Normal developing vitreous fibers were very dense and homogeneous, running towards the lens not only from the vitreous base but also from the retina at the equator and at the posterior pole. These findings suggest that extension of extraretinal vasoproliferation in retinopathy of prematurity is induced by the framework of the developing vitreous.