This paper describes experience gained with 663 eyes of patients with diabetic vitreous hemorrhage treated by closed vitrectomy with a follow-up period of six months. Hemorrhages were nearly always associated with proliferative retinopathy (97%) and were thought to be caused always by ruptured proliferative vessels. Membrane removal was performed in 42% of all cases but in 60% when the retina was detached. The most common surgical complication was creation of retinal holes; more common when the posterior retina was detached (32%) than when it was attached (18%). The lens was removed in 73% of the cases. Eighty-two percent of the retained clear lenses remained clear at the six months follow-up period. Therefore, clear lenses should not be removed. Visual improvement could be achieved in 59% of the cases where the posterior retina was attached but only in 25% where it was detached resulting in an overall major visual improvement in 46% of the cases. If one adds to this group the cases that retained reasonable vision of 20/200 or better, the overall success rate was 51%. Posterior retinal detachment and rubeosis of the iris were the main factors for a bad prognosis. Forty-two percent of all eyes had at least some degree of rubeosis iridis. Most of the preoperative rubeotic eyes had rubeosis postoperatively (71%). Twenty-three percent of all eyes ended up with neovascular glaucoma.