Simple bone marrow fibrosis is seen in 10–30% of multiple myeloma (MM) patients. We investigated the incidence and characteristics of the bone marrow stromal alterations, in order to characterize the collagens involved by immunohistochemistry, and to evaluate the use of serum aminoterminal propeptide of type III procollagen (PIIINP) as a marker of marrow fibrogenesis and disease activity in MM. 34 consecutive patients with newly diagnosed MM were included prospectively, and followed for 12–30 months. Compared with the findings in 15 normal individuals we found increased interstitial deposits of collagen III in 48% of MM patients, whereas deposits of collagen I were not increased. Interstitial fibrosis appeared to be restricted to areas of severe plasma cell infiltration, but it could also have a more dispersed presentation in the severely infiltrated marrow. There was a high co-distribution of collagen III fibrils and reticulin fibres. Serum PIIINP levels were elevated in most patients, and in the follow-up study serum PIIINP showed a good correlation with the response to treatment. Patients with resistant or progressive disease had continually elevated levels of PIIINP. In most patients with responsive disease serum PIIINP normalized, and we observed no relapses in patients who had normal serum PIIINP levels. Other patients who responded to treatment by reduced M-component level, but had persistently elevated serum levels of PIIINP, had either early relapses or developed progression of osteolytic lesions in spite of unchanged M-component levels. Therefore an elevated serum PIIINP during treatment might indicate an active malignant clone. Serum PIIINP does not simply follow the M-component, but gives further information of potential therapeutic value.