Concentrations of copper (Cu) and ceruloplasmin in serum were measured serially in 49 preterm infants with mean (+/- SEM) birth weights of 979 +/- 33 g and gestational ages of 28.4 +/- 0.3 weeks at three, six, nine, and 12 months postpartum. Serial radiographic studies showed 17 infants with (group A) and 32 infants without (group B) rickets or fractures. Cu and ceruloplasmin concentrations in serum also were measured in 21 healthy term infants (group C) with birth weights 3668 +/- 98 g at three, six, and 12 months postpartum. Analyses of covariance of serial changes in these serum variables--taking into account such potential covariates as differences in gestational age, birth weight, initial weight and length, changes in weight and length during the study, the duration of parenteral nutrition, and increased enteral copper intake--showed both groups of preterm infants had significantly lower concentrations of Cu in serum up to age six months and ceruloplasmin up to age three months (P less than 0.001) when compared with term infants. By one year of age, Cu and ceruloplasmin concentrations in serum in all groups had increased significantly (P less than 0.001), into the adult range, and were not significantly different among groups. These data document a maturational lag in copper metabolism in small, preterm infants. Changes in concentrations of Cu and ceruloplasmin in serum were significantly correlated (r = 0.92, P less than 0.001) but were not significantly different between preterm infants with and without rickets or fractures at each age.