A hypothesis that the development of functional disability in rheumatoid arthritis (RA) would be more rapid at the beginning than in established disease was studied in a group of 82 patients with RA referred to the Rheumatology Unit of Nancy, France. The relationship between disease activity, articular destruction and functional disability indicated a significant interaction (moderating effect) of disease duration. Two different models are proposed to explain functional disability. A model for early RA (less than 5 years duration) includes biological activity, and a model for established disease (more than 5 years) includes disease duration, extraarticular lesions and radiographic damage. These models could be useful in designs of therapeutic trials.