The significance of IgM, IgA and IgG rheumatoid factors (RF) for the prediction of radiological progression, and as process variables during follow-up, was evaluated in a three-year prospective study of 149 patients with early rheumatoid arthritis (symptoms < 1 year at study entry). The occurrence of IgA-RF and IgG-RF at study entry without simultaneous occurrence of IgM-RF, and the seroconversion from RF-negative at entry to RF-positive during follow-up appeared to be unusual. A significant correlation was found between each of the RF-isotype levels at entry and radiological progression after three years. However, no significant prognostic value of IgA-RF and IgG-RF could be demonstrated if analysed in combination with IgM-RF, initial disease activity (as measured by C-reactive protein level), initial radiologic score, HLA-DR4 and HLA-DR2. Although IgM-RF levels generally reflected the course of disease activity and did so better than IgA-RF and IgG-RF levels, their clinical significance as process variables appeared to be limited compared to C-reactive protein.