Serum carboxy-terminal propeptide of human type I collagen (PICP) concentrations, as a marker for bone formation, and urinary pyridinium (Pyd) cross-link concentrations, as a marker of bone resorption, were determined in 66 healthy infants aged 1-18 months who are being studied longitudinally. We hypothesized that there would be a positive correlation of growth velocity, increase in bone area, and bone mass accretion rates with PICP and Pyd cross-link concentrations. Since osteocalcin is currently used as a marker of bone formation, serum osteocalcin concentrations were also measured. Mean serum PICP and urinary Pyd cross-link concentrations were significantly greater than adult concentrations. Future growth velocity, increase in bone area, and bone mass accretion rates were not associated with PICP, Pyd cross-link, or osteocalcin concentrations. Growth velocity during the 3 months preceding sample collection correlated with serum PICP, Pyd/kg, and osteocalcin concentrations (r = 0.474, p < 0.001; r = 0.379, p < 0.001; and r = 0.516, p < 0.001, respectively). Previous increase in bone area correlated with serum PICP concentrations (r = 0.359, p = 0.01). The relationship between the infant's previous bone mass accretion rate and PICP was of borderline significance (r = 0.281, p = 0.055). In summary, normative data for PICP, Pyd cross-link concentrations, and parameters of bone growth are provided for infants 1-18 months of age and indicate that these markers reflect past and current bone metabolism and may be helpful in monitoring bone disorders in infants.