In order to define the range of serum copper concentrations in preterm infants and to determine the effect of growth upon these values, serial serum copper concentrations were measured in 26 preterm infants over their first six weeks of life. Fourteen healthy, growing preterm infants (Group I) had mean serum copper concentrations below 32 microgram/dl throughout the study. Clinical and hematologic signs of copper deficiency which responded promptly to the oral administration of copper sulfate were noted in five of these 14 infants. Twelve ill preterm infants (Group II-A), who received parenteral nutrition without supplemental copper and had slow rates of growth, had mean serum copper concentrations above 50 micrograms/dl after the first week of life. Seven surviving from Group II-A (Group II-B) had a decrease in mean serum copper concentrations to values similar to those in Group I after two weeks of oral feedings and resumption of normal growth. Our findings suggest that preterm infants who have normal growth while receiving oral feedings are at significant risk for developing copper deficiency.