We reviewed the data on thirty-six supracondylar fractures of the femur (in thirty-four patients) that occurred after total knee arthroplasties that were done between April 1974 and December 1981. Patients who had osteoporosis, rheumatoid arthritis, one or more previous arthroplasties of the knee, or inadvertent breeching of the anterior aspect of the femoral cortex at operation appeared to be particularly at risk for a supracondylar femoral fracture. Malalignment of the component could not be implicated as a cause. Twenty-six fractures (in twenty-five patients) were treated by non-operative methods. Seventeen of them (65.4 per cent) healed and required no surgical treatment. Fourteen of the seventeen were followed for more than two years; they had no significant difference in the knee score and lost less than 10 degrees of motion. The nine remaining knees required revision of the arthroplasty because of non-union in four knees, malunion in two, loosening of the component in two, and extension lag in one. At an average of forty months after revision, the nine knees were rated as having one excellent, four good, three satisfactory, and one failed result. In contrast, only three of the five fractures that were treated by early open reduction and internal fixation had a satisfactory result, and one of them required a second bone-grafting procedure. One patient died perioperatively and another required an above-the-knee amputation because of sepsis. Of the three fractures that were initially treated by external fixation, one had an excellent and two had a good result at an average of forty-five months after fracture. We have found that supracondylar fractures that occur after total knee arthroplasty can be managed by either traction or application of a cast, or both, which usually results in healing of the fracture and a satisfactory outcome of the arthroplasty. Patients who have a poor arthroplasty result after non-operative treatment of the fracture usually can undergo a revision arthroplasty with the expectation of a satisfactory outcome. Operative treatment of the fracture should be reserved for patients who do not have osteopenia and in whom stable fixation can be achieved, for those who demand a highly functional arthroplasty, and for those in whom adequate closed reduction cannot be maintained.