In 78 patients with myelofibrosis with myeloid metaplasia (MMM) the serum procollagen III peptide activity (s-PIIIP) had a higher mean value than in 22 normal adult volunteers (22.5 v. 10 ng/ml). Nevertheless, 21.8% of the patients had s-PIIIP within the 95th percentile of the normal control group (16 ng/ml): with respect to those whose concentrations exceeded this limit, patients with normal s-PIIIP levels were younger (55.8 v. 61.2 years), had a higher Hb value (12.3 v. 10.5 g/l), a lower serum ferritin level (106 v. 464 micrograms/l) and a higher platelet count (390 v. 216 x 10(9)/l). In the overall patient population, s-PIIIP was significantly higher in those with symptoms of active disease (fever, sweating, weight loss) than in subjects with non-active disease (28 v. 16.9 ng/ml). At univariate analysis s-PIIIP correlated (at the 5% level) with increasing WBC, serum ferritin and number of transfusions and with decreasing Hb and platelet count. At multivariate analysis increasing WBC, serum ferritin and age proved to be independently associated with s-PIIIP. No relationship was found between the s-PIIIP level and morphometric grading of bone marrow fibrosis, megakaryocyte number, or lymphoid infiltration. Longitudinal studies showed that s-PIIIP increased with disease progression. The conclusion of the study is that s-PIIIP correlates more with overall disease activity than with the extent of bone marrow fibrosis.