BACKGROUND--Diabetes affects the vascular ocular circulation. Colour Doppler imaging allows for simultaneous two dimensional anatomical imaging and Doppler evaluation of blood flow velocity; with this technique ocular blood flow velocity can be evaluated in normal and diabetic patients. METHODS--A prospective study compared blood flow velocity in ocular vessels (ophthalmic artery, posterior ciliary arteries, central retinal vessels, and vortex veins) of 25 patients with proliferative diabetic retinopathy and 30 age matched normal subjects using a colour Doppler imaging unit (Toshiba Sonolayer SSA-270 A with a 5.0 MHz PLF-503 ST phased array scanning head). RESULTS--The diabetic patients had lower blood velocities than the volunteers. There were significant differences in ophthalmic artery; systolic (p < 0.01), diastolic (p < 0.001), mean velocity (p < 0.05), and central retinal artery; systolic (p < 0.001), diastolic (p < 0.001), mean velocity (p < 0.05). No significant correlations were found between age and blood velocities. CONCLUSION--Ocular blood flow velocity was decreased in diabetic patients with proliferative diabetic retinopathy.