Trace mineral metabolism has reached a new importance and received a new stimulus with the development of total intravenous feeding. A syndrome of acute zinc deficiency has been described in patients receiving intravenous feeding with pure amino acid infusates. Although the rapid response to zinc therapy makes it tempting to assume that the syndrome we have recognized during total parenteral nutrition is one of pure zinc deficiency, it is very likely that this is an oversimplication. The subtle relationships between zinc and other metals, such as calcium, copper, cadmium, and selenium, need further clarification and may account for some patients with low serum zinc not developing the expected clinical manifestations. Copper deficiency also occurs, but its importance is not yet as clearly defined as that of zinc deficiency.