Choroidal malignant melanomas were examined after unsuccessful brachytherapy with ¹⁰⁶Ru/¹⁰⁶Rh ophthalmic plaques. The histologic findings were compared with those obtained in non-irradiated melanomas similar in size and location. Both groups revealed no significant clinical difference apart from the lower average age of the irradiated patients. Some of the histologic features (scar formation, changes in the distribution of cell types, rate of mitosis) represented reactions to irradiation. No single histologic feature was strongly correlated to the irradiated group. The phenomenon of radioinsensitivity observed in some malignant melanomas cannot be fully elucidated using conventional histologic methods.