Using a radioimmunoassay the serum concentration of the N-terminal propeptide of type III procollagen (P-III-P) was measured in 35 patients with chronic myeloproliferative disorders, including idiopathic myelofibrosis (n = 10), osteo-myelosclerosis (n = 4), transitional myeloproliferative disorder (n = 5), polycythaemia Vera (n = 10) and chronic myelogenous leukaemia (n = 6). The normal range in 35 healthy controls was 4.9–11.7 ng/ml. The serum concentration of P-III-P increased with increasing degrees of bone marrow reticulin fibrosis. By contrast, almost normal levels were detected in osteomyelosclerosis with an indolent clinical course, in which an excessive deposition of mature collagen fibres was found, representing mainly type I collagen. These observations indicate that the serum P-III-P level is positively correlated to the degree of bone marrow reticulin fibrosis, whereas levels are near normal in patients with osteomyelosclerosis and stable disease. Measurement of serum P-III-P may be a useful indicator of disease activity in rnyelofibrosing conditions.