The authors present the results of evaluation of platelet aggregation by means of an automated system (on line detection of platelet aggregation curves) in 80 patients with diabetes mellitus type I. After global analysis no significant changes were found between controls, patients without diabetic nephropathy, and patients with incipient and clinically manifested nephropathy. However, as a result of our data, the area below the aggregatory curve is minimal in the group of patients with clinically manifested diabetic nephropathy. Additional changes of platelet aggregation were observed after dividing the followed set of patients dividing into homogenous subgroups according to their sex. The sensitivity of platelets after induction by ADP was found to be lower in male diabetics than in male controls. Likewise the area below the aggregatory curve and the transmittance of absolute maximum of platelet aggregation was lower in female diabetics than in female controls. In both cases diabetic nephropathy could have participated in platelet sensitivity changes. The authors' findings in the followed group of patients when compared with the described platelet hyperaggregability in diabetes approves the possibility of the platelet aggregatory polymorphism being present in these patients. With respect to these findings the examination of the platelet aggregation and its general parameters exactly justifies the usefulness of the antiaggregatory therapy. (Fig. 4, Ref. 15.)