The concept of malignancy of the erythron was reexamined by characterization of erythropoiesis in a 6-year-old boy with erythroleukemia. Erythroblastic and myeloblastic activities were monitored with bone marrow and peripheral blood examinations at three-to four-week intervals for 21 months. Fluctuations in myeloblastic proliferation bore no relationship to the profile of erythroid proliferation. In contrast, a direct relationship between severity of anemia and magnitude of erythroid hyperplasia was demonstrated, and a physiologic relationship between serum erythropoietin and the hemoglobin concentration was observed. Though qualitatively abnormal, erythropoiesis in erythroleukemia is regulated by physiologic control mechanisms.