1. The ten cases of leukemia occurring among 3480 atomic bomb survivors in this study present convincing evidence of the high incidence of leukemia in atomic bomb survivors.

2. In all but one case blood studies were carried out prior to the development of pathognomonic features of leukemia. One case of reticulum cell leukemia was encountered on routine examination in a preclinical stage but with primitive or blast cells in the peripheral blood smears.

3. Of the ten cases, seven were acute or subacute leukemia and three were chronic myelogenous leukemia.

4. Observations on the acute cases of leukemia were of limited value because of the long interval in most cases between routine blood studies and the development of leukemia. However, in one case, examination ten weeks before clinical onset revealed no hematologic or physical findings suggestive of leukemia.

5. In contrast to acute leukemia, the long preclinical stage in two cases of chronic myelogenous leukemia allowed repeated hematologic and biochemical studies which were of considerable interest.

6. In these cases the long latent period and prolonged preclinical course were similar to those reported in cases of chronic myelogenous leukemia occurring spontaneously and following exposure to x-ray and other forms of radioactivity.

7. In the preclinical stage, disturbances in the red cells and platelets as well as leukocytosis with immature neutrophilic granulocytes were noted. The consistent presence of increased numbers of basophils, many of them atypical, was an outstanding feature of the peripheral blood smears in early myelogenous leukemia.

8. In two cases of preclinical myelogenous leukemia, studies on the separated leukocytes of the peripheral blood gave extremely low values for alkaline phosphatase.

9. Certain features of chronic myelogenous leukemia following irradiation suggest that the disorder may be due to a loss of growth-promoting or regulating factors.

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