We correlated predicted radiation distributions in 29 patients with uveal melanoma (treated with 5,000 to 8,000 rads of helium ion therapy) with fundus photography and visual field deficits. The tumor was entirely within the treatment region in all patients. The fraction of the predicted high-dose and low-dose regions corresponding with an absolute scotoma was fitted by a linear regression model (r2 = .721) with three independent variables: dose region, initial condition, and time since therapy. The defect in the maximum-dose region increased at an average rate of 0.270/year. This differed (P = .002) from the rate in the penumbra region: (0.104/year). The rate in the penumbra differed (P = .01) from that in the predicted low-dose region, where scotoma did not increase with time. The pattern and time course of visual field loss in the different radiation dose regions tended to support the conclusion that isodose calculations are accurate. The data were consistent with the hypothesis that visual loss secondary to irradiation results primarily from radiation vasculopathy and not from direct damage to photoreceptors or ganglion cells. Development of an absolute scotoma may require a dose in excess of 5,000 rads.