The present study was performed to evaluate whether information concerning synthesis and degradation of type I collagen in multiple myeloma (MM) as obtained by serum analyses of C-terminal propeptide of type I procollagen (PICP) and the C-terminal telopeptide of type I collagen (ICTP) may be useful in evaluating the development of osteolytic bone destruction. Serum N-terminal propeptide of type III procollagen (PIIINP) may give information about marrow fibrosis in MM. No data are available about MM and serum hyaluronan, another important component of bone marrow stroma. We examined 15 consecutive patients before treatment and 15 sex- and age-matched controls. We found highly significant elevations in serum ICTP (median 6.2 vs. 2.4 micrograms/L; P < 0.01), PIIINP (median 5.2 vs. 2.9 micrograms/L; P < 0.01) and hyaluronan (median 122 vs. 45 micrograms/L; P < 0.01). ICTP in serum correlated closely to bone morbidity (r = 0.69; P < 0.01). Furthermore, serum ICTP correlated highly significantly to serum PIIINP (P < 0.01) and serum beta 2-microglobulin (P < 0.01), whereas there was no correlation between hyaluronan and any of the collagen-derived peptides or beta 2-microglobulin. The MM group was followed for 9-25 months and analysis of survival data suggested that serum ICTP may be of predictive value (P < 0.05). We conclude that important changes in connective tissue metabolism occur in MM. ICTP in serum seems to be a noninvasive marker of bone morbidity and may be of prognostic value. Furthermore, elevation of hyaluronan in serum is common in MM, the significance of which is unknown.