Eighty-seven diabetics 8.5-26 years old (mean +/- SD 15.3 +/- 3.9) and 32 healthy non-diabetic controls 8-27 years old (13.9 +/- 4.3) were included in the study. They had had the disease for 2 months to 19 years (5.8-4.0 years). Seventy-two of the diabetic patients were HLA-DR type, 13 patients had DR 3 alone, 25 DR4, 33 DR 3,4 and 1 patient was neither DR 3 nor 4. The mean fluorescein concentration in the vitreous body 3.5-7 mm from the retinal surface at 60 min after intravenous administration of fluorescein was 15.5 +/- 11.9 ng/ml in the diabetics and 7.2 +/- 3.7 ng/ml in the non-diabetic controls (P less than 0.001). The diabetics still in partial remission had an almost normal blood-retinal barrier (BRB; 7.9 +/- 4.8 ng/ml) while about 55% of the diabetics beyond remission had impaired barrier function. Abnormal leakage was found in some patients who had had diabetes for less than 2 years and also before the onset of puberty. The incidence of abnormal leakage increased with increasing age and duration. There was a positive correlation between fluorescein leakage and the blood glucose level at the onset of diabetes (P less than 0.01). There was no statistically significant relationship between specific HLA-DR types and abnormal leakage. A defect BRB was significantly correlated with poor short-term metabolic controls, expressed as glucosuria index during the last week before examination with vitreous fluorophotometry. Prospective studies will show whether abnormal BRB gradually leads to irreversible retinopathy.