Vitreous and aqueous humor fluorescein concentrations were measured one hour after graded intravenous fluorescein was given to 20 juvenile diabetics, ages 20 to 40, with and without retinopathy, and to 12 controls of similar age. Vitreous fluorescein concentrations were significantly higher in diabetics, indicating breakdown of the blood-retinal barrier. Mean vitreous fluorescein values were 10.66 +/- 0.65 for the diabetics and 4.28 +/- 0.37 ng./ml. for the controls. Breakdown of the blood-retinal barrier was also confirmed in diabetics under the age of 20 without retinopathy. The blood-aqueous barrier was similarly altered in diabetics. Vitreous fluorophotometry quantitatively measures breakdown of the blood-retinal barrier, possibly the earliest detectable ocular vascular abnormality in juvenile diabetic patients.