Six hundred sixteen eyes with recent severe diabetic vitreous hemorrhage reducing visual acuity to 5/200 or less for at least 1 month were randomly assigned to either early vitrectomy or deferral of vitrectomy for 1 year. The proportion of eyes with visual acuity of 10/20 or better was higher in the early vitrectomy group than in the deferral group throughout the 4-year follow-up period. Up to the 18-month visit, the early group had a higher proportion of eyes with visual acuity of no light perception. An increased chance of obtaining good vision with early vitrectomy was clearly present in the type I diabetes group, particularly in patients who developed severe vitreous hemorrhage after less than 20 years of diabetes, a patient group tending to have more severe proliferative retinopathy. This advantage was not found in the type II diabetes group, in which patients were older and tended to have less severe retinopathy. The findings of this and previous Diabetic Retinopathy Vitrectomy Study reports support early vitrectomy in eyes known or suspected to have very severe proliferative diabetic retinopathy as a means of increasing the chance of restoring or maintaining good vision.