The main difference between monoclonal gammopathy of undetermined significance (MGUS) and multiple myeloma (MM) is the presence of lytic bone destructions in the latter. About 20% of MGUS patients develop MM, and histomorphometric studies have shown disturbed bone turnover rates in some of these patients. This study was performed in order to evaluate whether serum analyses of the C-terminal telopeptide of type I collagen (ICTP), as a reflector of bone degradation, and of osteocalcin, bone-specific alkaline phosphatase (bAP) and the C-terminal propeptide of type I procollagen (PICP), as markers of bone formation, might give information on disturbances of bone metabolism in MGUS. Furthermore, serum N-terminal propeptide of procollagen III (PIIINP) might give information on disturbances in collagen III metabolism in the bone marrow. In the 35 patients examined, serum ICTP was elevated in 12 patients (34%), serum PIIINP elevated in 6 patients (17%), serum osteocalcin elevated in 11 patients (31%), serum bAP elevated in 6 patients (17%), and serum PICP elevated in 4 patients (11%). Serum ICTP correlated significantly with PIIINP (r = 0.72, p < 0.001), and with serum osteocalcin (r = 0.57, p < 0.001) and serum bAP (r = 0.51, p = 0.002). These findings indicate disturbances of bone turnover and affected collagen metabolism in some MGUS patients. Follow-up observation may reveal any prognostic value of these findings.