The increasing number of hip and knee arthroplasties implies a greater likelihood of periprosthetic fractures and need for successful treatment options. We asked whether in situ effective lengthening of the indwelling prosthesis by a custom-made slotted hollow intramedullary nail provided a reasonable alternative to the established internal fixation techniques and prosthesis exchange. Between 1994 and 2005, we treated 25 patients (four male and 21 female; average age, 80 years) with a hip or knee periprosthetic fracture using this technique. Preconditions included a well-fixed prosthesis with a conical tip. In 23 hip cases a retrograde femur nail and in two knee fracture cases an antegrade nail were used for in situ lengthening of a femoral hip or knee implant stem. In all 25 cases, we used a preoperatively manufactured custom-made implant; in 20 patients, we recommended immediate mobilization by partial or full weightbearing. Eighteen of 25 patients were followed a minimum of 7 months (mean, 25 months; range, 7-31 months). Three patients died and four were lost to followup. We observed fracture healing in all patients, but one female patient had subsequent prosthesis loosening. The major complication rate was 6% (one of 18). We believe effective lengthening of the indwelling prosthesis by a custom-made slotted hollow intramedullary nail is a reasonable option for treating periprosthetic femoral fractures.