The electro-oculogram in 52 patients with a suspected malignant melanoma of the choroid or ciliary body was plotted in a diagram constructed for the differential diagnosis of malignant melanoma, metastasis, naevus and retinal detachment. Thirty-one patients were diagnosed as suffering from malignant melanoma on clinical grounds (19 histologically confirmed). Twenty-six were classified correctly as a melanoma using our EOG probability score. Rupture of Bruch's membrane and tumor localization were of no influence on the EOG classification. Accompanying retinal detachment lowered the Lp/Dt-ratio significantly without affecting the Dt, as was also the case in tumors with a prominence equal to or greater than 6 mm when compared with smaller tumors. However melanomas were still classified correctly in the majority of the patients by means of EOG. We conclude that an acceptable differentiation can be made between melanomas, retinal detachments and naevi. Melanomas cannot be differentiated from choroidal metastases.