Whilst developing an elbow endoprosthesis, the joint forces were estimated for patients with rheumatoid arthritis. Elbow flexion strength of rheumatoid patients was found to be 45% of normal. Muscle strengths and limb geometry data were found by a dissection technique, which allowed joint forces to be calculated during flexion, extension and abduction efforts. Forces up to 2.4 kN were predicted to act on the distal humerus, with similar forces acting in both radius and ulna. The skeletal structure is well adapted to carry the predicted forces, and onlay-type prostheses are recommended for elbow replacement.