Serum levels of the aminoterminal type III procollagen peptide (S-PIIINP) have been used as markers of proliferative inflammation in rheumatoid arthritis (RA) and a prognostic significance has been suggested. To test this further we have measured S-PIIINP longitudinally for 2 years in 66 patients with definite RA and a disease duration of less than 2 years, and related the levels to clinical, biochemical, and radiographic findings. In this patient group the correlations between S-PIIINP and ESR and CRP, respectively, were higher than those obtained between S-PIIINP and articular indices, and markedly higher than in patients with RA of longer duration. Patients with normal mean levels of S-PIIINP during the study period had a significantly slower rate of radiographic progression than patients with elevated mean levels of S-PIIINP. ESR yielded in general higher correlations with the joint damage process than did S-PIIINP. The correlations between S-PIIINP and the joint damage scores increased with time. A multiple regression analysis showed that ESR explained most of the variance in joint damage progression over 2 years, but S-PIIINP added independent information. About one third of the variance could be explained by the two variables.