The aim of the study is to evaluate the role of electroretinography in diagnosis of central retinal vein occlusion CRVO. There are two forms of this entity, each of them having a different prognosis. While haemorrhagic form has a better outcome, with fairly good prognosis for vision, ischaemic form usually develops many complications: macular oedema, neovascularisation of the retina or optic disk, neovascular glaucoma, and possible blindness. Three months after the onset, when oedema and retinal haemorrhages are usually resolved, it is possible to perform fundus fluorescein angiography (FFA) and make differentiation between the two forms of the disease. However, neovascular glaucoma may challenge the vision even before the diagnosis of ischaemic form becomes possible by observing the fundus or by performing FFA. Trying to foresee the course of the disease, and thus to help a patient with panretinal photocoagulation, we performed electroretinography (ERG) in each patient just after the onset of the disease. The prospective study included 40 patients with CROV which lasted less than three months and without any complication. Two parameters were examined: scotopic "b" wave and photopic b/a ration. By ERG action potentials from the retina after its stimulation with light, are recorded. Scotopic "b" wave generates in bipolar layer from Muller's and bipolar cells. Photopic b/a ratio is a ration of two waves, "b" and "a" in photopic conditions and is a good indicator of saturation of the retina with blood and oxygen. Visual acuity, applanation tonometry, examination of the fundus after dilatation of pupils with Sol. Mydriacili were performed in each patient. Each patient was examined by ERG in scotopic and photopic conditions. The results were as follows: Scotopic b wave per se could not indicate potential complications, while b/a ratio was a good predictor of possible complications when its value was less than 1.25. CONCLUSIONS Electroretinography, which can be performed at any time after the onset of the disease may be a good indicator of retinal perfusion and oxygen saturation, by giving the ratio of b and a waves under photopic conditions. The status of Muller's cells and bipolar cells is reflected in the scotopic b wave. In a prospective study forty patients with CRVO of less than three months duration and without neovascularisation were studied with the use of ERG, both in scotopic and photopic conditions. Our results suggest that the b/a ratio may be a good predictor of the development of retinal, disk and iris neovascularisation by showing the degree of retinal ischaemia, while the scotopic b wave cannot be used for such evaluation. Predicting the new vessel development by ERG may save the useful vision and prevent a disastrous outcome, blinding and painful neovascular glaucoma by performing panretinal photocoagulation.