The authors examined the relationship of change in tumor height after proton beam irradiation with the risk of metastasis in 700 patients treated for uveal melanoma before July 1, 1986. Rates of change in tumor height were computed for each patient using follow-up ultrasonographic measures during the first 2 years after treatment. Risk of metastasis was evaluated separately in the first 2 years after treatment when tumor decline was assessed (concurrent metastasis), and 2 years or later after treatment (subsequent metastasis). Using Cox proportional hazards models to adjust for other known risk factors, tumors regressing rapidly were significantly more likely to metastasize concurrently with their regression (P less than 0.05). The opposite was found for subsequent metastasis: tumors with a slow annual decline were more likely to metastasize after 2 years after treatment (P less than 0.05). Substantial changes also were noted in the ability of previously described prognostic factors (largest tumor diameter, tumor height, ciliary body involvement, and patient's age) to predict early and late metastasis. Characteristics placing individuals at high risk of metastasis may change markedly with time after treatment.