Although osteosclerotic bone metastases are characteristic of prostate cancer, mixed metastases with a lytic component are not uncommon. Type I collagen is synthesised by osteoblasts and accounts for about 90% of the organic matrix of bone. We have used new specific immunoassays for PICP (carboxy-terminal propeptide of type I procollagen) and ICTP (cross-linked carboxy-terminal telopeptide of type I collagen) which allow simultaneous assessment of the synthesis and degradation of type I collagen respectively. Forty patients with bone metastases due to prostate cancer at the time of diagnosis were investigated with these methods. Twenty-three of them had sclerotic (S) and 17 had mixed metastases with sclerotic and lytic components (S + L) as assessed by radiographs. The concentrations of PICP and ICTP in serum as well as the activity of alkaline phosphatase (AP) were increased in all patients of the S + L group, who had more aggressive bone disease and a shorter survival than the S group (P < 0.017). The ICTP level was above the reference range in half of the patients in the S group, whereas the PICP and AP levels were elevated in 35%. Of the bone markers, only ICTP was of prognostic significance (P < .05). We conclude that ICTP and PICP give information about the type and activity of the skeletal metastases. In addition, ICTP predicts prognosis.