BACKGROUND Currently, there is little information available on the treatment and outcome of intraoperative periprosthetic humeral fractures that occur during shoulder arthroplasty. The purpose of this study was to report on the incidence, treatment, and outcome of, as well as the risk factors for, intraoperative periprosthetic humeral fractures. METHODS Between 1980 and 2002, forty-five intraoperative periprosthetic humeral fractures occurred during shoulder arthroplasty at our institution. Twenty-eight fractures occurred during primary total shoulder arthroplasty, three occurred during primary hemiarthroplasty, and fourteen occurred during revision arthroplasty. Nineteen fractures involved the greater tuberosity, sixteen involved the humeral shaft, six involved the metaphysis, three involved the greater tuberosity and the humeral shaft, and one involved both the greater and lesser tuberosities. All patients were followed for a minimum of two years. At the time of the latest follow-up, outcomes were assessed, radiographs were examined, and relative risks were calculated. RESULTS Over the twenty-two-year study period, the rate of intraoperative humeral fractures at our institution was 1.5%. All fractures healed at a mean of seventeen weeks. In the primary arthroplasty group (thirty-one patients), range of motion and pain scores improved significantly (p < 0.05) at the time of follow-up. In the revision arthroplasty group (fourteen patients), range of motion remained unchanged whereas pain scores improved significantly (p < 0.005). Transient nerve injuries occurred in six patients. Four fractures displaced postoperatively and were then treated nonoperatively; all four healed. Significant relative risks for intraoperative fracture were female sex, revision surgery, and press-fit implants (p < 0.05). CONCLUSIONS The data from the present study suggest that although intraoperative humeral fractures are associated with a high rate of healing, there was a substantial rate of associated complications, including transient nerve injuries and fracture displacement. Significant risk factors for intraoperative fractures include female sex, revision surgery, and press-fit humeral implants.