A randomized double blind trial was performed to investigate the effect of the platelet aggregation inhibitor ticlopidine on the rate of decline in renal function in diabetic nephropathy. Twenty-two patients with insulin dependent diabetes complicated by nephropathy completed the trial--11 on ticlopidine, and 11 on placebo for one year. Ticlopidine effectively reduced platelet aggregation in vitro. Renal clearance of 51Cr-EDTA declined from 39 +/- 10 to 30 +/- 13 ml/min per 1.73 m2 body surface in the ticlopidine group and from 42 +/- 9 to 39 +/- 13 in the placebo group. The difference in decline between the two groups was not significant. In the ticlopidine group renal function expressed as the slope coefficient for 1/S-creatinine per month remained the same as before the trial. It is concluded that although there is much evidence to suggest a role of platelets in the development or progression of diabetic nephropathy treatment with ticlopidine could not prevent this process.