Three methods are available for measurement of retinal blood flow: cinefluroescein angiography, measurement of mean transit time, and estimation of volume flow by laser-Dopler velocimetry. Cineangiography requires intra-arterial injection for volume flow measurement, and this greatly inhibits its use. Difficulties also arise in estimating the relation of the axial to the mean flow, since it is now known that the retinal flow is plug flow. Transit time measurement using fluroescein has recently been automated and thus allows for more accurate measurement and better estimation of volume flow. The problems of estimating vascular volume and allowing for leakage have not been overcome. Laser-Doppler velocimetry measures mean red cell velocity but its relation to volume flow has not been established. Using the mean-transit-time method, several authors have found reduced transit time and increased volume flow in patients with mild and no retinopathy, compared to normals and those with more severe lesions, This most probably is due to autoregulatory adaptation and, possibly, to hypoxia. In more severe retinopathy, flow is reduced and there are large areas of nonperfusion. These areas are the result of abnormality of the endothelial cells and also of the blood within the vessels, which probably coagulates more easily than in normals. These areas are of importance because they stimulate the growth of new vessels.