Following extensive bowel resection, a young woman experienced severe malnutrition; subsequent administration of parenteral nutrition precipitated the copper deficiency syndrome. This consisted of hypocupremia, subnormal ceruloplasmin levels, anemia, and severe neutropenia. The bone marrow was megaloblastic, vacuolated, and sideroblastic; granulocytic maturation was not observed beyond the myelocyte stage. Copper sulfate therapy was followed by a marked reticulocytosis, increase in hematocrit, and recovery of neutrophils. Additional studies indicated that both serum and urinary erythropoietin values were low; serum activity increased after copper supplementation. Abnormal granulopoiesis was demonstrated using the in vitro granulocyte colony assay. The patient's granulcoytic stem cells were normal on two occasions; however, mixing studies showed that culture of the patient's copper-deficient marrow with her copper-deficient serum yielded significantly reduced numbers of granulocyte colonies. Thus, copper appears to be a necessary element for normal hematopoiesis; lack of this trace element may result in ineffective erythropoiesis and granulopoiesis.