1. Observations made on the bone marrow smears of 36 patients with idiopathic hemorrhagic purpura and the correlation of the findings with the clinical picture are presented. The control series consisted of 50 patients with nonhemorrhagic conditions without blood dyscrasias.

2. The bone marrow in idiopathic hemorrhagic purpura is hyperplastic. There is a slight myeloid and erythroid immaturity and in some cases a slight eosinophilia and lymphocytosis.

3. The megakaryocyte counts ranged from 3 to 59 per 10,000 nucleated cells, with an average of 17. In the control series the megakaryocytes ranged from 1 to 54 per 10,000 nucleated cells, with an average of 16.

4. There appears to be no correlation between the number of megakaryocytes found in the marrow smears during the acute phase of the disease and the prognosis with or without splenectomy. There is also no apparent correlation between the number of megakaryocytes and the platelet response following splenectomy.

5. The megakaryocytes were classified as megakaryoblasts, immature, intermediate, or mature megakaryocytes, or as naked nuclei. The principal criteria for the differentiation of megakaryocytes are the granules and the presence or absence of granular platelets.

6. The differential megakaryocyte counts were made by examining 25 or more megakaryocytes. In the control series the mature megakaryocytes which are actively producing platelets are the predominant cells, whereas in idiopathic hemorrhagic purpura the intermediate cell without platelet production is predominant.

7. Marrow studies on 2 of our patients before and after splenectomy revealed a decrease in the relative number of megakaryocytes and an increase in the number of platelet-producing cells following operation.

8. No correlation was found between the eosinophil counts and the deaths, recurrences, or cures with or without splenectomy.

9. The principal value of the bone marrow examination in cases of suspected idiopathic hemorrhagic purpura is to exclude leukemia and aplastic anemia.

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