In normal subjects, mean platelet volume (MPV) has an inverse, nonlinear relation with platelet count, while platelet volume heterogeneity has a direct, nonlinear relation with MPV. Compared with the reference range established for normal subjects, patients with chronic lymphocytic leukemia, atherosclerotic heart disease, diabetes mellitus, and chronic undifferentiated schizophrenia had normal platelet volume mean and heterogeneity. Patients treated with cytotoxic chemotherapy for acute nonlymphocytic leukemia, patients with megaloblastic anemia, and patients with aplastic anemia had abnormally small platelets with increased heterogeneity. Patients with chronic myelogenous leukemia had abnormally large platelets with increased heterogeneity.