We evaluated the effect of insulin on platelet function, blood viscosity, and filterability in healthy subjects and in patients with Type 2 (non-insulin-dependent) diabetes mellitus. Fifteen diabetic patients were free from cardiovascular complications (group A), while the other 15 patients had both clinical and measured evidence of coronary or peripheral vascular disease (group B); 15 non-diabetic subjects served as controls. On blood samples taken without stasis, maximal platelet aggregation to 1.25 mol l -1 ADP, blood and plasma viscosity, and blood filterability were measured in basal conditions, and after incubation of blood, plasma or platelet-rich plasma with insulin at two physiological concentrations (120 and 480 pmol l -1 ). Compared with healthy subjects, the diabetic patients of group B had higher values of blood (p Ο½ 0.01) and plasma (p Ο½ 0.05) viscosity, and platelet aggregation response to ADP (p Ο½ 0.01), as well as lower values of blood filterability (p Ο½ 0.01). The diabetic patients of group A had values intermediate between normal subjects and the patients of group B. In non-diabetic subjects, insulin significantly decreased platelet aggregation and blood viscosity at low shear rates (22.5 s -1 ) (p Ο½ 0.01 for both), and had no significant effects on other parameters. In the diabetic patients of group A, insulin decreased blood viscosity at high (225 s -1 ) rates of shear (p Ο½ 0.01) and increased blood filterability (p Ο½ 0.01). The effects of insulin were not dose-related. In the diabetic patients of group B, none of the parameters evaluated was significantly influenced by insulin. Type 2 diabetic patients present many abnormalities of the rheologic properties of blood. The beneficial effects of insulin on platelet aggregation and blood viscosity are not evident in Type 2 diabetic patients, especially those with vascular complications and this may be relevant to the development of those complications.