Bone marrow fibrosis is known in myelomatosis and depends on the extent of plasma cell infiltration. The serum concentration of the aminoterminal propeptide of type III procollagen (PIIINP) has previously been reported to reflect fibrogenesis in the marrow in myelofibrosis. Here we followed 15 consecutive patients with newly diagnosed multiple myeloma with repeated PIIINP measurements during treatment with intermittent courses of melphalan and prednisolone. PIIINP was found to change with clinical behaviour of the disease, nonresponders and patients with recurrent disease having elevated values and the values in responders decreasing to the normal level or remaining there. Collagen fibres in plasma cell infiltrates of biopsies from bone marrow or skeletal tumours of these patients stained heavily with antibodies against PIIINP. Our results suggest that PIIINP works as a noninvasive indicator of bone marrow fibrogenesis. In multiple myeloma PIIINP is a sensitive, but not specific marker of disease course.