Reconstruction after resections for high-grade sarcomas about the knee in children and adolescents is a challenging problem because of the large soft tissue and skeletal defects, the effects of adjuvant therapy, and the potential for long-term use of the limb. One hundred sixteen patients, all 18 years or younger, with osteosarcoma or Ewing's sarcoma located between the middle femur and middle tibia, were treated with chemotherapy, resection, and allograft reconstruction. One hundred three patients with osteosarcoma and 13 patients with Ewing's sarcoma had 105 Stage II and 11 Stage III tumors. There were 72 osteoarticular grafts (39 femur, 33 tibia), 28 intercalary grafts (19 femur), seven allograft-prosthetic composites (all femur,) and nine allograft-arthrodeses (seven femur, two tibia). At latest followup, 49% of all of the allograft reconstructions were rated good or excellent, 14% were rated as fair, and 37% were failures. Sixteen percent had an infection develop. Twenty-seven percent of patients had a fracture, 34% had a nonunion, and 14 patients eventually required amputation. Reconstruction of large bone defects about the knee in young patients who are being treated with chemotherapy is difficult. Although complications significantly affect outcome, allografts are a viable option for reconstruction in children with high-grade sarcomas about the knee.