To the Editor:— Rubeosis iridis is a common ocular complication of diabetes mellitus. Its pathogenesis is no better understood than diabetic retinopathy. It is generally known that reducing the ocular pressure below normal, or systemic hypertension, exerts a deleterious influence on the evolution of diabetic retinopathy. Duane has suggested that this pressure differential between the inside and the outside of a normal retinal arteriole is a critical factor in determining the development of diabetic retinopathy. He defines this as the index of net vascular pressure (intra-arterial pressure/intraocular pressure) and the greater the index the more susceptible the retinal tissues are to retinopathy in diabetes.1 If this mechanism can be used to explain exacerbations of diabetic retinopathy, it would seem reasonable that it would also explain the aggravation of vessels in the anterior segment, manifested by rubeosis iridis. In the past six months, two diabetics were seen with a rather