A 15-year-old girl with a 3-month history of pain in her right knee was first seen in June 1990. Clinical examination revealed a soft-tissue mass over the medial aspect of the proximal end of the tibia. She was short of stature and had atrophic skin changes with hyperpigmentation and aplastic thumbs. Roentgenograms showed a destructive lesion in the proximal area of the right tibia, with a mixed sclerotic and lytic appearance (Fig. 1). The results of a chest roentgenogram and computed tomography (CT) scan of the lungs were normal. However, a bone scan showed increased uptake in the proximal area of the right humerus and the distal end of the left fibula. Roentgenograms showed a destructive sclerotic lesion in the proximal area of the right humerus and the distal end of the left fibula (Figs. 2, 3). A biopsy of the right proximal tibial lesion was performed.