Reticulated platelets (RP) were measured with an automated hematology analyzer (modified R-2000) in 287 healthy volunteers and in 212 patients with thrombocytopenia. In healthy volunteers, the RP was 0.48 +/- 0.26% in men and 0.48 +/- 0.32% in women. No significant difference in the RP values due to gender or age (21-60 years) was observed. Furthermore, the reverse correlation was observed between platelet counts and RP. The RP was high in patients with idiopathic thrombocytopenic purpura (ITP), those with high fibrinogen and fibrin degradation products (FDP), and those with high C-reactive protein (CRP), but low in patients after chemotherapy. The RP was highest in active phase of ITP, and relatively high in the partial remission phase of aplastic anemia. In patients after chemotherapy, the patients had a minimum phase of RP and then a maximum phase of RP before platelet counts increased. RP was significantly high in the maximum phase and significantly low in the minimum phase. The relationships between platelet count and RP were negatively correlated in patients with ITP, high FDP, or high CRP, but were not correlated in patients with aplastic anemia, liver disease, or after chemotherapy. These results show that RP reflects the pathology of thrombocytopenic disorders and the measurement of RP is useful for the differential diagnoses and analysis of platelet kinetics.