Although copper is an essential nutrient in the human diet, overt or severe copper deficiency is not a major public health concern in the United States. A marginal or borderline deficiency has been suggested by researchers, although the widespread nature of copper deficiency has yet to be established. Reports indicate that copper deficiency occurs secondary to gastric resection, unsupplemented total parenteral nutrition, high levels of zinc intake, or general malnutrition. An early clinical sign of copper deficiency is a reduction in the number of circulating neutrophils. Although copper is known to play a wide variety of roles in the organism and in the immune system, the molecular mechanism for copper-deficient neutropenia is not known. Very little data exist with which to examine this question. This paper will summarize our existing knowledge of the mechanism by which copper deficiency results in neutropenia. Although the data are scarce, analysis of this question will allow us to better understand additional molecular roles of this trace element, and, in turn, to promote an improved knowledge of immune cell functions and cellular differentiation.