Nine humeral fractures occurred subsequent to 499 shoulder arthroplasties that had been performed between December 1978 and November 1987 at the Mayo Clinic. The time from the arthroplasty to the fracture averaged thirty-nine months (range, eight to 101 months). Seven patients were women and two were men, and the average age was seventy years (range, forty-five to eighty-five years). The arthroplasties were performed for rheumatoid arthritis in five patients and for the sequelae of trauma in four. Six patients had advanced osteopenia, and two had had an ipsilateral total elbow arthroplasty. Six of the fractures were centered at the tip of the prosthesis; one fracture (type A) extended proximally, and five (type B) did not. The three remaining fractures (type C) involved the humeral shaft distal to the implant and extended into the distal humeral metaphysis. Four fractures healed with non-operative treatment. Two fractures that had unacceptable alignment were treated successfully with operative intervention. Three fractures that were treated with immobilization in a splint failed to heal; two of those fractures eventually united after a revision of the prosthesis and bone-grafting was performed, and one fracture remained ununited. Radial nerve palsy developed postoperatively in two patients, and it resolved within three months. Five patients had poor active motion before the fracture, and two of them had even less motion after the fracture was treated. Our experience suggests that long oblique and spiral fractures can be successfully treated non-operatively, provided that the skeletal alignment is acceptable.(ABSTRACT TRUNCATED AT 250 WORDS)