PURPOSE: We performed this study to determine the diagnostic significance and useful cut-off value of reticulated platelet, which can be used for discriminating the destructive thrombocytopenia from underproductive thrombocytopenia. METHODS: We evaluated 37 patients with thrombocytopenia who were admitted to the Wonju christian hospital from March to July, 1995. All patients were evaluated with bone marrow megakaryocyte count. We divided them into two groups, group 1 was consisted of underproductive thrombocytopenic patients and group 2 was consisted of destructive thrombocytopenic patients. We measured the peripheral blood reticulated platelet count by FACScan flowcytometry after thiazole orange staining. RESULTS: 1) Among 37 patients, underproductive group consisted of 21 patients and destructive group consisted of 16 patients. 2) There was no significant difference in platelet count between two groups (41.7 31.8x109/L vs. 41.0 29.1x109/L). 3) Destructive group showed higher reticulated platelet count than underproductive group (7.7 3.7% vs. 19.1 11.1% : p<0.01). 4) Ideal cut-off value of reticulated platelet count for the discrimination of two groups was 11%. Its sensitivity and specificity were 88% and 95%, respectively. CONCLUSIONS: We suggest that reticulated platelet count is closely correlated with thrombopoiesis of bone marrow. The measurement of reticulated platelet can be a simple and useful diagnostic tool to discriminate destructive thrombocytopenia from underproductive thrombocytopenia.