A new biomodular prosthesis was used for the treatment of a displaced fracture of the proximal part of the humerus in twenty-two shoulders in twenty-two patients. The fractures were classified according to the Neer system; there were thirteen four-part, five three-part, and four head-splitting fractures. There were fifteen women and seven men, and the mean age was seventy years (range, forty-nine to eighty-seven years). The hemiarthroplasty was performed an average of eleven days (range, one to forty-five days) after the injury. The deltopectoral interval was used in all patients, and the prosthesis was implanted with cement in twenty of the shoulders. All of the patients participated in a supervised program of rehabilitation. The patients were followed for an average of thirty-six months (range, twenty-six to forty-nine months). Twenty of the twenty-two patients had a good or excellent result. The active forward elevation averaged 119 degrees; external rotation, 40 degrees; and internal rotation, to the twelfth thoracic vertebra. All of the patients except for the two who had a poor result had satisfactory relief of pain. The two patients who had a poor result had a successful revision with a modular prosthesis of the same design. The modular head could be removed, enabling the surgeon to gain access to the glenoid and to adjust the soft tissues. The over-all scores correlated inversely with the age of the patients and the interval from the injury to the operation.(ABSTRACT TRUNCATED AT 250 WORDS)