A 10-year-old white boy presented with a diffuse pigmented lesion of the choroid of his right eye that had led to a partial secondary retinal detachment. Because of the fear of a diffuse choroidal melanoma (a lesion never before reported in an individual within the first two decades of life), the eye was enucleated. An incisional P-32 study, compared with an uninvolved region, was positive at 154% uptake. Pigmented tissue was discovered surrounding the distal meninges of the optic nerve, on the posterior surface of the eye, and within the orbital soft tissues adherent to the globe. This led to fear of extraocular extension of a malignant melanoma. Light microscopic examination of the enucleated globe demonstrated that the choroid was diffusely and massively thickened by polygonal, hyperpigmented tumor cells that were also present in compressed spindled forms between the lamellae of scleral collagen, in the scleral emissaries, on the surface of the eyeball, and in the distal optic nerve dura. The choroidal tumor cells had the features of those normally encountered in localized optic nerve head benign melanocytomas. Electron microscopy demonstrated the presence of large, well-melaninized melanosomes (but no macromelanosomes), confirming the diagnosis of a diffuse uveal melanocytoma. The differential diagnosis of this new entity and its relationship to previously reported pigmented lesions of the uvea is discussed.