The clinical and histopathologic features of 29 eyes from 28 patients with central retinal vein occlusion (CRVO) are reported. A fresh or a recanalized thrombus was observed in each eye. This study considers the temporal aspects of the cases, and it notes the different morphologic features of the occlusion. These observations explain most of the variability of the changes observed in previous reports. We believe these different features representthe various stages in the natural evolution of such a thrombus. The interval between CRVO and histopathologic study in our series ranged from six hours to more than ten years. Local and systemic factors were reviewed and were found to be important in the pathogenesis of thrombus formation. Local diseases with a predisposing effect on CRVO included: glaucoma, papilledema, subdural hemorrhage, optic nerve hemorrhage, and drusen of the optic nerve head. Associated systemic diseases included: hypertension, cardiovascular and cerebrovascular disease, diabetes mellitus, and leukemia with thrombocytopenia. A fresh thrombus in the CRVO was observed in three (10.3%), and a recanalized thrombus in 26 eyes (89.7%). Endothelial-cell proliferation was a conspicuous feature in 14 (48.3%) of the eyes. Chronic inflammation in the area of the thrombus and/or vein wall or perivenular area was observed in 14 (48.3%) of the eyes. Arterial occlusive disease was observed in seven eyes (24.6%). Cystoid macular edema was found in 26 (89.7%) of the eyes.