A patient presented with acute erythromyelosis (DiGuglielmo) which was developed after 3 yr of aplastic anemia. Aplastic anemia differed from the classical form, since erythroid cells and megakaryocytes were relatively preserved in the bone marrow. Treatment with androgens induced the increase of hematocrit and reticulocyte as well as general improvement. The sudden appearance of hemorrhagic syndrome due to thrombocytopenia was associated with aggravation of anemia and granulocytopenia. In the bone marrow, giant multinuclear proerythroblasts with bizarre nuclear morphology and PAS positivity with coarse granules was found. Serum erythropoietin (Ep) level was high. Bone marrow cells culture in vitro revealed two types of erythroid colonies: typical and giant multinuclear cells, both benzidine-positive. The number of colonies was irrespective to the Ep dose. "Autonomous" Ep independent growth of these colonies was also demonstrated. The number of colonies was more than 3 times higher per number of cells seeded when compared to normals, which indicated malignant proliferation and Ep independent growth. Treatment with 6-mercaptopurine and transfusions was without effect and the patient died after 15 days with signs of cerebral bleeding.