The permeability of the blood retinal barrier (PBRB) and the diffusion coefficient into the anterior chamber (kd) in 20 insulin dependent (ID) and in 11 non-insulin dependent (NID) diabetics with various degrees of retinopathy were determined by fluorophotometry with a 25% accuracy. The difference of PBRB and the kd values between the NID and the ID patients was not significant (p greater than 0.05 and p greater than 0.22). The mean PBRB and the mean kd values differed significantly from those of a healthy population (p less than 0.0013 and p = 0.025). A significant correlation was established between PBRB and diabetes duration (r = 0.55; p less than 0.01) but not between PBRB or kd values and metabolic control (HbAl) (p greater than 0.5) or creatinine clearance (p greater than 0.5). The time integrals of unbound plasma fluorescein in the diabetics between the time of injection and 1 hour later were comparable with those of healthy controls. The difference between the mean PBRB value of diabetics with advanced retinopathy and that of a healthy population was significant (p less than 0.003) but the difference for the mean values of diabetics with no or minimal background retinopathy was not (p greater than 0.2), indicating that PBRB values do not increase to an abnormal level while signs of diabetic retinopathy are absent on fluorescein angiography.