A 35-year-old man presented with insidious onset of severe sacral pain. Plain radiography, computed tomography, and magnetic resonance imaging revealed a large, locally invasive mass within the sacrum. Skeletal scintigraphy showed marked hyperemia and minimal peripheral osteoblastic activity of the sacral mass. An F-18 FDG PET study was performed for further assessment and clinical staging. The mass demonstrated high glucose avidity consistent with a high-grade tumor. Histologic examination confirmed the lesion to be a plasmacytoma. Solitary plasmacytoma of bone occurs predominantly in older patients involving the axial skeleton. This case is interesting in view of the large dimension, sacral involvement, the young age of the patient, and the appearance of this lesion across multiple imaging modalities.