We developed two new radiographic parameters-the humeral surface height ratio and ulnar surface height ratio-to precisely detect changes in the bony structure of rheumatoid elbows. Of the 59 patients with rheumatoid arthritis, 101 elbows were classified into four types (osteoarthritis, ankylosis, erosive, and resorptive) according to the radiographic appearance. Clinically, osteoarthritis type and ankylosis type were considered to be stable form, and erosive type and resorptive type were unstable form. Patients' clinical data and yearly radiographic changes in the bony structure evaluated by the humeral surface height ratio and ulnar surface height ratio were compared among the four types and between the two forms. There were significant differences between the two forms and among the three types except for the ankylosis type in yearly radiographic changes in the bony structure evaluated by the humeral surface height ratio and ulnar surface height ratio. Stable and unstable forms were distinguished by a cut-off point of 0.65 and 2.58 in yearly radiographic changes in the bony structure evaluated by the humeral surface height ratio and the ulnar surface height ratio, respectively. These parameters might be useful for monitoring the structural changes of the elbow joint in rheumatoid arthritis.