Mean platelet volume (MPV) and platelet distribution width (PDW) were measured using a Coulter counter (Model S-plus), in newborns, normal children, and healthy adults and in various clinical conditions. MPV was significantly increased in the patients with idiopathic thrombocytopenic purpura (ITP) and iron deficiency anemia (IDA), whereas in those with aplastic anemia and leukemia it was normal. The MPV of the patients with ITP decreased as the platelet count increased, and it became normal when the patients' platelet counts reached the normal range. In acute poststreptococcal glomerulonephritis (APSGN), renal failure and cyanotic congenital heart disease, the MPV was significantly increased. In the pregnant women with preeclampsia, the MPV showed a significantly higher value than in normal spontaneous vaginal delivery (NSVD), spontaneous premature rupture of the membranes (SPRM) and abortion. In the adults, with rheumatic heart disease, angina pectoris myocardial infarction and diabetes mellitus the MPV was significantly increased over that of the control group.