The light and electron microscopic findings are reviewed in two patients who had eyes enucleated for peripheral choroidal malignant melanomas. Preoperatively, cystoid macular edema was documented by fluorescein angiography in the melanoma-containing eye in both patients. Intracytoplasmic swelling (edema) of the Müller (glial) cells is the anatomical basis for the macular edema. Intercellular (extracellular) collections of fluid probably are late, endstage results of the process that result form prolonged, excessive, intracellular edema, cell death and disruption. The process probably rests on an ischemic basis, as evidenced by severe changes in the microvasculature. In the one patient in whom the optic nerve was available for study, marked intracellular swelling (edema) of glial cells in the lamina choroidalis of the optic nerve head was present, associated with compression of the adjacent axons. The nearby temporal, parapapillary retina also showed edema of Müller cells, and compression of the nerve fibers (ganglion cell axons), suggesting a more widespread process than was clinically evident. Again, severe changes were present in the microvasculature, both in the optic nerve and parapapillary retina. The underlying cause of the microvasculature changes that lead to ischemia, perhaps an intrinsic pharmacologic agent, is yet to be found.