The platelet aggregation and serum lipids were evaluated in three groups, that is normal control, diabetes mellitus and liver cirrhosis. The platelet aggregation was made a comparison between one and another group using the plasma exchange method. Briefly, after centrifugation of platelet-rich-plasma, the supernatant was replaced with the platelet-poor-plasma (PPP) of another group. Platelet aggregation was performed by method of Born. In conventional method, platelet aggregation of diabetic patients was slightly increased but that of cirrhotic patients was decreased. Whereas, after plasma exchange, the aggregation of normal controls was slightly increased in diabetic PPP. Hyper LDL and hypo HDL-cholesterolemia of diabetic patients may be plasma factors on platelet aggregation.