[en] According to clinical experiences of radiation accidents during the past two decades, utilization of measured hematologic changes as a direcrt indicator of the severity of radiation injury provides important information for diagnosis and prognostic evaluation in individual cases. Hematologic changes can be described in terms of prognostic categories based on the possible outcome of the acute radiation syndrome. The five categories suggested by Wald according to the grade of severity. By the actual application of this category to our experience of the 1971 Chiba accident of exposure to irridium 192, it was proved that the estimated dose was well correlated to the value by cytogenetic analysis and physical estimation used of thermo-luminescence phenomena. In hematological parameters, a decrease of lymphocytes occurs whithin 24 hours after the exposure. The level of this early lymphopenia is regarded as one of the best indicators of severity of radiation injury. For the decision of therapeutic procedures, however, the total granulocyte count and platelet count are more valuable to exclude severe infection and bleeding symptoms occurred one month after the exposure. The limitation of the approach by hematologic data must exist in the case exposed in a non-uniform fashion. To overwhelm this difficulty, the application of rapid marrow scanning by short-lived RI such as ⁵²Fe is expected and the bone marrow imaging by magnetic resonance studies is more exciting. For more sensitive and technically easy-drived methods detecting hematologic injury, our new method of detecting micro-nucleus in polychromatic erythroblasts from cultured erythroid colonies from peripheral blood is now developing. Preliminary data have shown the sensitivity of this method is comparable to the cytogenetic study of pheripheral lymphocytes. (author)