The value of nucleolar organizer regions in uveal melanoma. The Collaborative Ocular Melanoma Study Group. Silver staining of nucleolar organizer regions is an objective method for evaluating the malignancy of a variety of tumors. We studied 126 ciliochoroidal melanomas, three coincidental nevi that occurred in eyes with melanomas, and one magnocellular nevus collected from the Collaborative Ocular Melanoma Study to determine the effectiveness of the silver-stained nucleolar organizer region technique in assessing the malignant potential of these tumors. Malignant lesions demonstrated higher mean silver-stained nucleolar organizer region counts (4.347) than benign nevi (1.855) (P less than or equal to .0001). Among malignant melanomas, mixed-cell melanomas had slightly higher counts than spindle-cell melanomas (P less than or equal to .0001), but this difference was not important clinically. Results were also compared to other histopathologic variables, which disclosed correlation of silver-stained nucleolar organizer regions with mitoses and tumor size. Comparison with computerized cytomorphometric analyses of prognosis also disclosed significant correlation. This technique may prove to be a useful adjunct in the assessment of malignancy and treatment response of uveal melanomas. 