The mainstay of local control of primary bone malignancies in the skeletally immature has been amputation or, in selected cases, rotationplasty. The development of expandable endoprostheses has permitted an alternative approach for local control in the growing child. Between January 1985 and December 1987, 12 skeletally immature patients with primary malignant bone tumors were treated with extremity reconstruction with cemented custom-expandable endoprostheses after wide resection of their lesions. All patients were observed until death (four) or revision (two) with a minimum two-year follow-up period for the survivors (average, 3.1 years). Seven patients have undergone a total of 11 expansions and one patient was lengthened with a revision-expandable prosthesis. Four patients have not needed expansion. Eight patients have had a total of ten complications. Seven of the ten complications (70%) were prosthesis related and associated with failure of the expansion mechanism. The Musculoskeletal Tumor Society (MSTS) overall rating was good to excellent in seven patients (58%), fair in three (25%), and poor in two (17%). In five distal femoral arthroplasties and one total femoral arthroplasty where the tibial bearing component was cemented through the physis, tibial and epiphyseal growth was observed to be normal and equal to the nonoperative side. This suggests that partial central epiphyseal and physeal ablation does not cause physeal arrest. Although the high rate of expansion mechanism failure necessitates redesign, preliminary results suggest that expandable endoprostheses do offer an alternative to amputation and rotationplasty as a means of local control and extremity reconstruction in children with primary malignant bone tumors.