1. The hematologic syndrome called refractory anemia with hyperplastic bone marrow, aregenerative anemia, chronic bone marrow failure, pseudo-aplastic anemia, and many other terms, has been separated into five types on the basis of clinical and hematologic studies on 23 patients carried out over a period of 16 years. These groupings are probably highly artificial but are made to facilitate presentation and description. As more is learned of the chemistry of the bone marrow cells in patients with refractory anemia, a more satisfactory classification will be possible.

2. Type 1 is characterized by bizarre chromatin in the normoblasts, mast cell hyperplasia, hemosiderosis or hemochromatosis and a tendency for spontaneous remissions to occur.

3. Type 2 is at first typified by complete or almost complete erythroid aplasia. Hyperplasia of erythroid cells with maturation arrest and bizarre chromatin pattern may occur later, and finally spontaneous remission may appear with considerable frequency. Mast cell hyperplasia, thymoma and hemosiderosis have been noted.

4. Type 3, associated with exogenous toxins and the preleukemia state, is characterized by pancytopenia and a bizarre chromatin pattern in the normoblasts. Some of these patients may he classified as "DiGuglielmo syndrome."

5. Type 4 also is typified by pancytopenia and hyperplasia of bone marrow but with cells of normal appearance. These patients responded partially or completely when splenectomy was performed, and probably represent a variant of the "hypersplenism" syndrome.

6. Type 5 is refractory megaloblastic anemia with typical cytologic changes evident in all cell types similar to those found in pernicious anemia.

7. Hypotheses are proposed to explain these various types of anemia on the basis of abnormalities in the metabolism of nucleic acids, particularly DNA. Deficiencies of metabolites, inhibition of metabolic reactions by exogenous or endogenous toxins, or by immune mechanism in which DNA serves as haptene, are possible explanations.

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