One hundred and seventeen consecutive patients with refractory dysmyelopoietic anemia (RDA) were followed and studied over 6 yr. All RDA cases had at least two marrow cell lines involved with dysplasia and usually all three. Morphologically, the series could be divided into 55 cases that had primarily “erythroid” hyperplasia and 62 cases with primarily “myeloid” dysplasia. There was a significantly higher number of acute leukemias in the myeloid group, 21/62 (33.9%), than in the erythroid group, 7/55 (12.7%). Of the 117 cases, 64 had marrow culture studies performed using implanted diffusion chambers (DC) in an irradiated rat host. There was a highly statistically significant correlation between the development of leukemia and abnormal growth in DC during the RDA stage.