Gliclazide has been reported to possess the properties of preventing the progression of diabetic retinopathy and of controlling blood glucose levels. This report describes a long-term comparative clinical trial of this agent to assess its efficacy against diabetic retinopathy. One hundred and fifty-nine NIDDM patients with no retinopathy or with simple retinopathy entered this trial. One hundred and nineteen patients receiving other sulfonylurea agents were randomly allocated to two groups (G: gliclazide, SU: other sulfonylureas). Forty patients continued to be treated with diet alone (D group). Finally a total of 60 patients, that is, 21 patients in the G group, 19 patients in the SU group, and 20 patients in the D group, were followed with funduscopic examinations for more than 4 years. The results are summarized as follows. (1) Distribution of background factors between the two drug therapy groups was balanced, but in the D group more male patients and relatively milder cases were involved than in the drug therapy groups. (2) Fasting blood glucose control in the three groups was not significantly different. (3) Funduscopic deterioration was observed less frequently, though not significantly, in the G group than in the other groups. (4) Progression to preproliferative retinopathy was significantly less frequent in the G group than in the SU group. Thus, gliclazide seems to have additional properties compared with other sulfonylurea drugs in preventing deterioration of diabetic retinopathy, and particularly in preventing progression to proliferative retinopathy.