We conducted this study to determine whether the small-diameter, press-fit stem of a novel, noninvasive expandable endoprosthetic device implanted in the limbs of 6 pediatric patients with Stage IIB osteosarcoma affected the growth of the physis through which the stem was inserted. Local control of the tumor necessitated resection of the involved growth plate, and implantation of the device required penetration of the adjacent uninvolved bone, through the growth plate, by the stem of the device. We measured longitudinal growth and the rate of growth of the adjacent uninvolved bone in the salvaged limb and of the equivalent bone in the unoperated contralateral limb. In all cases but one (in which the patient's growth in the contralateral limb appeared complete), longitudinal growth continued in the limb into which the device was implanted: the adjacent uninvolved bone in the salvaged limb grew by an average of 2.4 cm, and the equivalent bone in the unoperated contralateral limb grew by an average of 2.3 cm. We conclude that implantation of a smooth, press-fit stem through the central portion of the uninvolved adjacent physis does not result in growth retardation or arrest.