The authors have treated 255 uveal melanomas with helium ion radiation. Twenty-three eyes have been enucleated because of complications and five eyes have been obtained at autopsy. We have evaluated 27 of these eyes. Neovascular glaucoma (10 eyes), painful keratitis (6 eyes), continued tumor growth (4 eyes), and vitreous hemorrhage (2 eyes) were the major complications of treatment that led to enucleation. The degree of tumor necrosis correlated with the size, pigmentation, and anterior extent of the tumor. It did not correlate with the interval from irradiation or with the amount of tumor shrinkage. Mitotic figures were extremely rare in treated tumors, suggesting that the tumor cells have lost their ability to cycle.