Markers of types I and III collagen turnover were measured in serial blood samples in 16 patients with a Colles' fracture. The collagen markers were the carboxy-terminal extension peptide of type I procollagen (PICP) and the amino-terminal extension peptide of type III procollagen (PIIINP). Significant increases were found of PIIINP within 1 week and of PICP within 2 weeks. This sequential appearance of PIIINP and PICP was found to be in agreement with the appearance of types III and I collagen during early fracture healing as demonstrated in previous animal experimental studies. PICP had levelled off after 9 months, whereas PIIINP remained elevated. Osteocalcin, a serum marker of osteoblast activity, increased within 1 week and levelled off after 9 months. Correlations between the change in osteocalcin and those in PICP and PIIINP, respectively, were weak. These new biochemical markers may prove relevant as non-invasive markers of normal and pathological fracture healing in humans.