To clarify the diabetic complications mediated by increased platelet activity, we undertook a study of the mean platelet component (MPC), as determined by an automated hematologic analyzer (ADVIA 120®, Bayer). Prothrombin time (PT), activated partial thromboplastin time (aPTT), and fibrinogen were also measured to investigate blood viscosity abnormalities. MPC was determined in 100 healthy controls and in 100 diabetic patients, the latter of which were subdivided into no diabetic retinopathy (DR) (n = 25), nonproliferative DR (n = 30), and proliferative DR (n = 45) groups. The mean MPC level was 26.9 g/dl in the control group and 22.5 g/dl in the diabetic patients (p < 0.05). PT and aPTT were similar for the diabetic patients and the controls; however, their corresponding fibrinogen levels were significantly different between the two groups (3.26 ± 1.14 g/L vs. 4.21 ± 2.35 g/L, p < 0.05). Our results suggest that platelet hyperfunction in diabetic patients may be implicated in the pathogenesis of diabetic retinopathy.