The authors studied the mitotic activity of 21 choroidal and ciliary body melanomas treated by external beam radiotherapy (2000 rad in 5 fractions over 5-7 days) before enucleation and 21 case-by-case matched nonirradiated uveal melanomas. The Wilcoxon signed ranks test and McNemar's fourfold table test were used to compare the mitotic activity of the irradiated and nonirradiated tumors. The results of these analyses suggest that pre-enucleation external beam radiotherapy substantially reduces the mitotic activity of posterior uveal melanomas (P less than 0.005). This information supports current efforts to mount a successful randomized clinical trial of pre-enucleation radiotherapy in high-risk patients with choroidal and ciliary body melanomas.