Copper is an essential trace element that is required for a number of enzymes which are necessary for normal metabolic function. Metabolic balance studies have demonstrated that daily copper losses are approximately 1.3 mg/day. In order to remain in copper balance, the average adult male must consume a diet which contains at least 2 mg copper/day. It has been assumed that most diets satisfy this requirement because of the ubiquitous presence of copper in most foodstuffs. Recent studies, however, have shown that dietary copper may often fall below the estimated daily needs. Nevertheless, clinically evident copper deficiency has been documented in only a few situations. Of these disorders. Menkes' syndrome has been most intensively studied. This x-linked abnormality is associated with copper deficiency due to impaired gastrointestinal copper absorption. However, the clinical disorder cannot be corrected readily with copper replacement, thus suggesting that Menkes' syndrome may reflect more than simple copper deficiency. Nutritional copper deficiency appears to be well documented in two situations: (1) the newborn, usually premature, undergoing rapid growth on a diet poor in copper, and (2) the patient maintained on total parenteral nutrition for long periods of time without copper supplementation. In both of these situations, anemia and neutropenia are the most striking hematologic abnormalities associated with copper deficiency. Sideroblastic changes as well as nuclear maturation defects observed in erythroid precursors. However, suggest that there is an effect of copper deficiency on the hematopoietic system which cannot be explained solely by defective plasma iron transport.