Retinal changes resembling those of background diabetic or hypertensive retinopathy commonly occur in leukemia, whereas retinal neovascularization is rare. When neovascularization does occur, it is usually the result of hyperviscosity caused by a greatly increased number of circulating leukocytes. A 42-year-old woman with diabetes mellitus developed chronic myelocytic leukemia and peripheral retinal neovascularization. The hyperviscosity leading to the neovascularization was probably caused by an increased number of circulating platelets.