This 2-year-old boy presented with fever and right arm pain. A. A posterior scintigraphic image shows prominent uptake in the proximal aspect of the ulna and reduced uptake throughout the remainder (arrow), including lack of normal uptake in the distal physis. B. A sagittal T1-weighted MR image shows multiple areas of hypointensity (arrow) in the ulna. C, D. A large amount of hyperintense subperiosteal fluid (arrows) is visible surrounding the ulna on these fat-suppressed T2-weighted images. The elevated periosteum is hypointense. The medullary portion of the ulna is hyperintense due to edema. There is extensive hyperintense edema in the soft tissues of the forearm. Effusions are present in the wrist and elbow (R, radius). E. An axial contrast-enhanced fat-suppressed T1-weighted image shows hypointense subperiosteal fluid (arrow), enhancement of the elevated periosteum, and extensive enhancement of adjacent soft tissues. Surgical exploration demonstrated pus in the subperiosteal space and in the ulnar medullary cavity. Cultures grew Methicillin-resistant Staphylococcus aureus.