Hand radiography of 200 patients with rheumatoid arthritis RA was repeated after an interval of 12 months. Twenty joints including ten metacarpophalangeal joints, eight proximal interphalangeal joints and two interphalangeal joints of the thumbs were assessed. The number of joints with erosion was recorded, being referred to as Erosion Score. Additionally, each joint was graded on a zero to five point scale and the gradings of the individual joints were summed to form a score referred to as Damage Score. The progression in these scores during 12 months was recorded. The radiological joint damage was found to develop rapidly during the first years after disease onset and to assume a slow rate of increase after 9 years of the disease. The results imply that the disease duration is a critical feature for therapeutic studies. We suggest that patients with disease duration of less than 36 months only are suitable for therapeutic trials of RA and that the follow-up time should be at least 18 months if radiological assessment is used.