A 1 3-year-old boy had pain and swelling in his left knee. The frontal and lateral radiographs of the left knee show a lytic lesion in the proximal tibial metaphysis (Figs. 1A and 1 B). Sonography shows multiple fluid-fluid interfaces (Fig. 1C). MR images show the lesion to be of heterogeneous signal intensity, expanding, obliterating, and extending anterior to the cortex of the tibia (Fig. 1 D). The T2-weighted images show high signal intensity within the lesion; the multiple fluid-fluid fluid levels previously documented on sonography. Note high-intensity signal from fluid.