We retrospectively studied the results of arthrodesis of the knee with a modular titanium intramedullary nail that couples at the knee. The study group consisted of thirteen patients who had a malignant tumor around the knee, five who had failure of a total knee arthroplasty, and three who had a locally destructive benign tumor about the knee. All of the patients were followed for a minimum of two years. Through a single incision at the knee, one nail was inserted retrograde into the femur and the other, antegrade into the tibia; the two nails were joined at the level of the knee by a conical couple and were secured with locking screws. The diameters of the nails were different, to accommodate the dissimilar sizes of the tibial and femoral intramedullary canals. A solid osseous fusion was achieved in nineteen (90 per cent) of the twenty-one patients (sixteen who had had resection of a tumor and three who had had a failed arthroplasty), at an average of 8.4 months (range, three to nineteen months) after the operation. One patient had a delayed union, but fusion was achieved after additional bone-grafting. Of the sixteen patients who were available for clinical and radiographic evaluation at the time of the study, fifteen were satisfied with the over-all outcome and thirteen had either less pain or the same amount of pain as they had had preoperatively. There were no mechanical failures of the implant and no recurrences of tumor. Complications occurred in eight (38 per cent) of the twenty-one patients: three patients had a stress fracture, three had a peroneal nerve palsy (one of which was transient), one had a superficial wound infection, and one had reflex sympathetic dystrophy.