Aqueous flare intensity was measured with the laser flare-cell meter in 231 eyes of diabetic patients and 31 eyes of normal age-matched controls. Diabetic patients were divided into four groups based on the degree of retinopathy: (1) non-retinopathy, 42 eyes; (2) background retinopathy, 72; (3) preproliferative retinopathy, 23; and (4) proliferative retinopathy, 94. There was no significant difference between the normal controls and the non-retinopathy group, whereas the rest of the diabetic groups showed significantly higher flare values than did normal controls (P less than 0.001). Flare intensity increased with the progression of retinopathy. Our results demonstrate that clinical use of the flare-cell meter enables the quantitative assessment of blood-aqueous barrier function in diabetics and suggest that diabetic iridopathy, as one of the manifestations of diabetes in the anterior part of the eye, exists even in the early stages of this disease and progresses in parallel with retinopathy.