One of the major complications of implantation of a massive frozen cadaveric allograft in the treatment of a tumor is fracture of the allograft. To determine the incidence, risk factors, appropriate management, and results of treatment of this complication, the records of the Orthopaedic Oncology Unit of the Massachusetts General Hospital were reviewed. Forty-three patients were identified in whom a tumor had been treated with an allograft that had subsequently fractured. The over-all incidence of fracture was almost 16 per cent. When the several risk factors (age and sex of the patient, stage and site of the lesion, and so on) for the forty-three patients who had a fracture were compared with those for the rest of the series, the only correlation was the incidence of non-union at the site of the host-donor junction, which was significantly higher in the patients who had a fracture. The mean time to fracture was 28.6 months after the operation. Three types of fractures occurred: Type I (two patients), rapid dissolution of the graft; Type II (twenty-two patients), fracture of the shaft, which was observed more frequently in male patients and which occurred a mean of 27.6 months after the operation; and Type III (nineteen patients), fragmentation of the joint, which usually occurred later (a mean of 31.6 months postoperatively) and was found more frequently in female patients.(ABSTRACT TRUNCATED AT 250 WORDS)