In spite of the rather high rate of complications, it is clear that in selected cases of proliferative diabetic retinopathy with massive long-standing vitreous hemorrhage, significant improvement can be accomplished by pars plana vitrectomy. However, the number of eyes recovering 20/40 vision or better remains small (7% in our series at 9 to 15 months postvitrectomy), and consideration needs to be given to means to improve these figures. Whether vitrectomy at an earlier stage of the retinopathy can accomplish this is the subject of an upcoming cooperative study under the sponsorship of the National Eye Institute.