In order to evaluate the applicability of the laser flare-cell meter to diabetic oculopathy, we measured aqueous flare and aqueous "cells" in 84 diabetic eyes of 84 patients and 50 normal control eyes of 50 age-matched subjects. Using fluorescein angiography, diabetic retinopathy was divided into background retinopathy (11 eyes), preproliferative retinopathy (38 eyes), and proliferative retinopathy (35 eyes). In diabetic eyes, the occurrence of both aqueous flare (0.73 +/- 0.39 mg/ml human albumin equivalent) and aqueous cells (mean 2.96, range 0-35.7 cells/0.075 mm3) was significantly greater than in the normal control group (flare 0.14 +/- 0.06 mg/ml, cells 0.39, range 0-2 cells/0.075 mm3, P < 0.0001 and P < 0.0002, respectively). The flare values also showed differences between the diabetic subgroups, with flare values being higher in more advanced stages of diabetic retinopathy. No significant correlation could be found between the flare values and the kind of diabetic therapy, the duration of the diabetes, and the number of or the time interval since previous retinal laser coagulations. A flare value of more than 0.5 mg/ml was found to represent probably a "critical value" indicating a tendency towards preproliferative and or proliferative changes. The laser flare-cell meter is a valuable instrument for noninvasive, quantitative assessment of alterations of the blood-aqueous barrier in diabetes. The increase of flare values seems to parallel the progression of diabetic retinopathy.