The most abundant protein in bone is type I collagen. During type I collagen formation two extension peptides from both ends of the procollagen molecule, carboxy- and aminoterminal propeptides (PICP and PINP), are liberated in equimolar concentrations into the circulation. Type I collagen carboxyterminal telopeptide (ICTP) is formed during bone collagen breakdown and is liberated into the circulation. Serum concentration of the propeptides reflect bone formation, and the concentration of the telopeptide, bone resorption. We evaluated the usefulness of these bone remodelling markers in diagnosing and monitoring metastatic bone disease in breast cancer patients. Serum concentrations of ICTP, PICP and PINP were measured and the PICP/PINP-ratio calculated in 25 patients with bone metastases, 12 patients without metastases and their age matched healthy controls. S-ICTP and S-PINP were significantly higher in metastatic patients (p = 0.0001 and 0.02 respectively), and the S-PICP/PINP-ratio lower (p = 0.002) than in controls. S-PICP in metastatic patients did not differ significantly from that of controls. ICTP values in patients without metastases also differed from those of controls (p = 0.01). The clinical sensitivity for diagnosing metastatic bone disease was 56% for ICTP, 24% for PICP, 30% for PINP and 52% for PICP/PINP ratio. The clinical specifities were 93%, 100%, 98% and 91% respectively. During follow-up the changes in the marker values were parallel to the behaviour of the disease. We conclude that these markers alone are not sensitive enough for diagnosis, but they seem to be of use in detecting bone metastases and monitoring the activity of bone disease.