Collagen type I is the sole collagen type found in bones and tendons. Carboxyterminal propeptide, deriving and cleaved from procollagen type I (PICP) during collagen synthesis, is delivered into the blood, where it can be measured. According to current knowledge, PICP correlates with bone collagen synthesis and bone formation rate. Elevated serum levels of PICP in patients with Paget's disease, compared with normal subjects and correlated with serum alkaline phosphatase (Alk.Ph.), have been previously described. Thus, PICP may be a valuable marker of bone formation. PICP, serum Alk.Ph., serum bone Gla protein and 24-h urinary hydroxyproline:creatinine ratio have been measured in 47 cancer patients: 27 with predominantly osteolytic lesions (5 myeloma, 15 breast, 3 lung, 2 kidney, 1 bladder, 1 thyroid) and 20 with predominantly osteoblastic lesions (18 prostate and 2 breast). The higher levels of PICP were noted in patients with osteoblastic or mixed metastases. In the entire group of patients, a statistically significant correlation between PICP and bone Gla protein (r = 0.57; P < 0.001), PICP and Alk.Ph. (r = 0.80; P < 0.001), and bone Gla protein and Alk.Ph. (r = 0.44; P < 0.01) was noted. In those patients with osteoblastic metastases we observed a significant correlation only between PICP and Alk.Ph. (r = 0.62; P < 0.003). During chemotherapy, 13 of 20 patients with osteoblastic metastases who achieved objective response or stable disease showed a more rapid and significant decrease in PICP with respect to the other bone markers. Serum PICP level could be considered a good marker of osteoblastic activity.