Serum bone alkaline phosphatase (BALP), serum carboxy-terminal propeptide of type I procollagen (PICP) and serum bone gla protein (BGP) as markers of bone formation, serum carboxy-terminal telopeptide of type I collagen (ICTP) as a marker of collagen resorption and fasting molar ratio of urinary calcium to creatinine (CaCr) and serum parathyroid hormone (PTH) were determined in two groups of cancer patients: 48 with advanced or metastatic disease with negative bone scan and 174 with bone metastases categorised as having lytic, mixed or blastic lesions and with more or fewer than or equal to three sites involved. In patients without apparent bone involvement, bone formation markers were rarely elevated. Conversely, serum ICTP was frequently found to be supranormal, showing it to be a non-specific marker for early detection of bone metastases. As expected, values of bone formation markers progressively increased in patients with lytic, mixed and blastic lesions, but ICTP levels did not show any differences according to the types of bone appearances, confirming previous reports of elevated osteoclast activity also in patients with apparent blastic lesions. Serum PTH increased significantly in patients with lytic compared with patients with mixed and blastic appearances, paralleling the bone formation markers, but CaCr showed the opposite pattern. These data are compatible with calcium entrapment in the bone in patients with increased osteoblast activity. This so called'bone hunger syndrome'is further confirmed by the finding that in the subgroup of blastic appearances CaCr diminished whereas both ICTP and PTH increased according to the extent of tumour load in the bone.