Central retinal vein occlusion (CRVO) is a common cause of sudden visual loss and can occur at any age. Accurate diagnosis is essential, as differentiation of ischemic and nonischemic forms of the disease has prognostic importance. The introduction of color Doppler flow ultrasound has, for the first time, allowed visualization of the retrobulbar vessels; the central retinal artery and vein are easily identifiable. The authors conducted a prospective, blinded trial and compared images and waveforms from 20 patients with CRVO with those from 20 age-matched healthy volunteers. No difference in velocity indexes from the central retinal artery was detected between the patients' affected and unaffected eyes or between the affected eyes and control eyes. A marked reduction in venous velocity was recorded in the central retinal vein in the affected eyes when compared with that of the unaffected patient and control eyes. There was no difference in venous flow between the unaffected patient and control eyes. These initial results suggest that color Doppler flow imaging may have a role in the diagnosis of CRVO. Color Doppler flow imaging is non-invasive, is easy to perform, is well tolerated by patients, and allows temporal assessment.