A group of eighteen patients was followed prospectively to document the development and natural history of collateral vessels. A second group of 42 patients was examined on a single occasion to provide a survey of the incidence and associations of collaterals. The time of appearance of collaterals varied between 2 and 8 months. The highest incidence of collaterals was in the macular vein occlusions. An increase in collateral vessel formation was seen where there was failure of recanalization of the occluded vein. No significance in the incidence of collaterals was seen in relation to age, site of obstruction, or hypertension. However, an increase in the incidence of collaterals was seen with time.