A total of 149 eyes with retinoblastomas were examined histopathologically to determine the types and frequencies of mechanisms inducing glaucoma. The most common mechanism was iris neovascularization with secondary peripheral anterior synechiae formation. In such cases, the tumor was almost always located at the posterior pole with involvement of central retinal vessels and with growth of a highly vascularized tumor mass into the vitreous. The second most common mechanism was massive exudative retinal detachment causing pupillary block and angle closure. In such cases, tumor growth was present within and under the retina. The third most common mechanism was a combination of the aforementioned mechanisms. Uveitis and/or necrotic tumor tissue in the anterior chamber as the sole mechanism inducing glaucoma was infrequently seen.