Bidirectional laser Doppler velocimetry and monochromatic fundus photography were used to investigate retinal circulation in 26 normal subjects and in 51 insulin-treated diabetic patients with no retinopathy, background retinopathy (BDR), proliferative retinopathy (PDR), and patients with PDR who had been treated by panretinal photocoagulation (PDR-PRP). Measurements were obtained from main temporal retinal vessels in each eye. Venous diameter at the site of laser Doppler velocimetry measurements was significantly greater than normal in patients with BDR and PDR. The maximum velocity of red blood cells was significantly lower than normal in eyes with diabetic retinopathy. Calculated volumetric blood flow rate, however, was not significantly different from normal in eyes with no retinopathy, BDR, and PDR, and was significantly decreased from normal in patients with PDR who had been treated by panretinal photocoagulation. A comparison between this technique and the fluorescein dye dilution technique is provided.