We measured mean platelet volume (MPV) in patients with diabetes mellitus, compared with MPV in non-diabetic control subjects. Mean MPV was significantly increased in the diabetic subjects (8.9 +/- 0.07 fL, mean +/- SEM) compared with non-diabetic subjects (8.0 +/- 0.05) (p < 0.001). Since platelet size is a determinant of platelet function, with larger platelets being more reactive per unit volume, we believe platelets may play a part in the micro- and macro-vascular complications of diabetes mellitus.