Hydroxyurea treatment of myeloproliferative disorders. Macro-megaloblastic blood and bone marrow changes.

Blood and bone marrow changes induced by continuous low-dose hydroxyurea treatment are described. A linear increase in mean red cell volume was observed after onset of therapy. The entire normocyte population was replaced by abnormally large erythrocytes within 150 days. The bone marrow morphology changed in megaloblastic direction. Bone marrow iron stores and number of sideroblasts increased, findings compatible with ineffective erythropoiesis. Serum folate and cobalamin levels remained normal. These morphologic changes might cause confusion when examining blood or bone marrow samples from patients treated with hydroxyurea.