Type I collagen is the main collagen type found in mineralised bone. Specific immunoassays for PICP (carboxyterminal propeptide of type I procollagen) and ICTP (cross-linked carboxyterminal telopeptide region of type I collagen) allow simultaneous assessment of the synthesis and degradation of type I collagen in serum samples, respectively. Our aim was to find out whether these metabolites of type I collagen are useful markers for following bone turnover and evaluating treatment response in multiple myeloma, which is a good model disease of excessive osteolysis. Fifteen consecutive patients were studied before and throughout their treatment. Samples for serum PICP and ICTP were collected before starting each treatment course of melphalan and prednisolon. Response to treatment was evaluated by following the changes in M protein and bone roentgenograms. The disease was progressing in four and regressive in 11 patients, but in four of these a recurrence occurred. In nonresponders the ICTP concentration was permanently elevated despite treatment. In responders both increased or normal levels of ICTP were initially observed, but they returned to or remained in the reference interval during treatment. The ICTP concentration increased upon recurring disease. There was a strong correlation between the extent of bone lesions and ICTP. There was no correlation between ICTP and PICP, the latter mainly remaining within the reference range, a finding that suggests no change in bone formation. ICTP was a significant predictor for survival in this patient group (P less than 0.05). We conclude that ICTP is a specific and sensitive marker for bone resorption. Simultaneous use of serum ICTP and PICP offers an additional and easy means to follow bone turnover and evaluate the response to therapy in multiple myeloma.