In this study, we explored the methodology and clinical utility of reticulated platelets analysis. Reticulated platelets were measured using flow cytometry in peripheral circulation with thiazole orange (TO). We chose fixed platelet-rich plasma as the sample since it has been known to give stable results. The percentage of reticulated platelets in 7 patients with idiopathic thrombocytopenic purpura (ITP) was significantly increased (8.41±5.35%; mean±SD), compared with that in 21 normal subjects (1.92±1.27%). The value in 16 patients with liver cirrhosis was slightly below normal (1.86 ± 0.85%). These findings suggested that thrombopoiesis was not promoted under the condition of hypersplenism or of abnormal pooling, and that any elevation in the percentage level of reticulated platelets could be clinically useful in the diagnosis of ITP.