PURPOSE: To determine the causes of non-traumatic non-diabetic vitreous hemorrhage (NDVH) and to report the visual and anatomical outcomes and complications of vitrectomy for this condition. METHOD: In a retrospective case series, records of patients who had undergone vitrectomy for non-traumatic NDVH over a ten year period at Labbafinejad Medical Center, Tehran-Iran with at least six months of follow up were reviewed for causes of the condition and outcomes of surgery. RESULTS: From 1993 to 2003, 50 eyes of 49 patients (51% male) with mean age of 62.7±10.3 (range 35-87) years underwent vitrectomy for non-traumatic NDVH. Preoperatively, mean best-corrected visual acuity (BCVA) was 2.36±0.52 LogMAR and relative afferent pupillary defect was positive in 91.1% of the eyes. Mean BCVA increased significantly to 1.38±0.72 LogMAR at six months (P < 0.0001). Causes of non-traumatic NDVH included: branch retinal vein occlusion (56%), central retinal vein occlusion (16%), choroidal neovascularization (12%) and posterior vitreous detachment with retinal break, Eales' disease, familial exudative vitreoretinopathy and Terson's syndrome (each in 4%). The most common causes of poor visual outcomes were: macular pigmentary derangement (26%), optic atrophy (16%), severe lens opacity (12%) and epiretinal membrane (8%). CONCLUSION: Despite the significant increase in VA following vitrectomy, irreversible macular or optic nerve pathology limits significant improvement in central visual acuity in several cases of non-traumatic NDVH. Vascular accidents were the most common cause of this condition.