Reticulated platelets (RP) and large platelets (LP) were measured by an automated hematology analyzer (modified R-2000) in 287 healthy volunteers and 131 patients with thrombocytopenia or thrombocytosis. RP was significantly higher in patients with idiopathic thrombocytopenic purpura (ITP), especially in active phase, while RP was markedly lower in patients with essential thrombocytosis (ET) or chronic myelocytic leukemia (CML). LP was significantly higher in patients with ITP, especially in active phase, while LP was markedly lower in patients with aplastic anemia (AA), ET, or CML. In ITP, RP and LP were significantly higher in patients positive for anti-glycoprotein (Gp) IIb/IIIa antibody. RP and LP were poorly correlated with platelet-associated IgG (PAIgG). RP and LP were poorly correlated with plasma thrombopoietin levels, and negatively correlated with platelet count. These results show that RP reflects the pathology of thrombocytopenic disorders, and that measurement of RP is useful for the differential diagnosis and analysis of platelet kinetics.