In the 3-year period during which patients were enrolled into the Collaborative Ocular Melanoma Study, 413 eyes with clinical diagnoses of choroidal melanoma were examined histopathologically as of December 31, 1989. Four hundred eleven of these eyes were found to be diagnosed correctly. One eye, removed after preoperative external beam radiation, was found to have a hemangioma. The second eye, removed after radioactive iodine plaque placement, was described as a magnocellular nevus (melanocytoma) by four of the five pathologists on the Collaborative Ocular Melanoma Study Pathology Review Committee. The Collaborative Ocular Melanoma Study misdiagnosis rate of 0.48% is the lowest ever reported. The major challenge with regard to posterior uveal melanomas is no longer that of correct diagnosis but rather determination of the optimal treatment.