The authors describe a patient with acute rheumatic fever and polyarthritis in whom scintigraphy unexpectedly identified Tc-99m MDP uptake in the diaphyses of both tibiae. A dramatic rise in antistreptolysin-O titer and rapid resolution of tibial abnormalities paralleled marked articular improvement. Magnetic resonance imaging demonstrated a pattern consistent with marrow edema in the area of abnormal Tc-99m MDP accumulation. This finding has not been previously described in acute rheumatic fever, and it was suspected that the changes in the tibiae resulted from subclinical diaphyseal hyperemia from the inflammatory process observed in the contiguous joints.