Fourteen cases of hemiarthroplasty for four-part fractures of the proximal humerus were reviewed. Pain relief was satisfactory, but function was limited, mainly due to loss of glenohumeral abduction despite electromyographic proof of actively contracting abductors in all cases. Analysis of special radiographs of nine cases showed a direct relationship between the clinical results and the "humeral offset", or distance between the geometric centre of the humeral head and the lateral aspect of the greater tuberosity. This offset affects the lever arms of the glenohumeral abductor muscles. The implications for surgical technique and for the design of shoulder prostheses are discussed.