Out of 1300 eyes enucleated between 1966-1974, 300 subsequently revealed histologically-proved malignant melanomas of the uvea. In 264 cases the clinical diagnosis was correctly made. In 36 cases the i.o. malignant melanomas were clinically unsuspected. The incorrect clinical diagnoses included: secondary glaucoma (30), retinal detachment (2), iritis (2), and end/panophthalmitis (2). The clinical symptomatology and morphology of the secondary glaucoma caused by the i.o. melanomas did not differ from that due to other, non-neoplastic etiologies. In almost all of the 36 cases the eyes revealed unilateral severe opacity of the media. It is important that one consider an intraocular melanoma in cases of secondary glaucoma, retinal detachment, or intraocular inflammation in which the etiology is uncertain. This is also true in cases of blind, painful eyes in which there is no obvious etiology. Early diagnosis is of vital interest to the patient because this group, in which the malignant melanoma was unsuspected, may be, as suspected in this study, is characterised by extreme aggressivness and invasive capacity. One third of these eyes revealed extension of tumor through the sclera and optic nerve. Also these tumors often revealed more malignant cell types.