Vitreous seeds in retinoblastoma were studied clinically and pathologically. Vitreous seeds were found in 36.6% of retinoblastoma eyes. When tumors involved more than three quadrants of the retina and vitreous seeds were present, the prognosis was poor. When the tumor involved less than one-quarter of the retina, vitreous seeds were rare. Vitreous seeds were found most frequently in cases of undifferentiated tumor cells and endophytum type of proliferation. Although most vitreous seeds were necrotic tumor cells, some were almost intact tumor cells which were apt to be situated along blood vessels. The blood vessels in vitreous seeds had no pericytes and were derived from the tumor itself. Thus it is possible that tumor cells in the vitreous body can migrate to the anterior segment of the eye. Some tumor cells in vitreous seeds had much cytoplasm which contained mitochondria, ribosomes, fibrils, centrioles, and cilia with a presumed photoreceptor outer segment and intercellular junctions. These cytoplasmic features are very similar to those of neuroepithelial-type retinoblastoma cells. Undifferentiated cells were necrotic. Calcium deposition was found mainly in the necrotic cytoplasm of the tumor cells and occasionally on the chromatin granules of the nucleus. This may provide evidence that calcium can be bound to DNA to form radiopaque masses. No calcium-producing cells were found. Vitreous seeds contain a small number of almost intact tumor cells which are neuroepithelial in type, but most cells are necrotic. Although tumor cells may migrate to the anterior segment of the eye along or through blood vessels, the presence of vitreous seeds in itself is not always a bad prognostic sign. The prognosis is probably more closely related to the extent of the invasion of a tumor associated with vitreous seeds.