ADP-induced platelet aggregation was investigated in patients with diabetes mellitus at various stages of the development of retinopathy. Two parameters were used to evaluate the aggregation activity of platelets: the degree of aggregation and the dysaggregation percentage. There was revealed a direct correlation between the platelet hyperaggregation and the degree of diabetic retinopathy. A sharp elevation of the degree of aggregation and low dysaggregation percentage (up to its complete absence) were noted in patients with proliferating retinopathy; this could be supposed to be due to the supervention of the activating action of the altered vascular wall on the platelet aggregation. No reduction of platelet hyperaggregation under the effect of a brief treatment with such drugs as prodectin and dicion was revealed.