A 77-year-old woman was admitted with intermittent abdominal dull pain. Hypochromic anemia, hypergammaglobulinemia, and elevated C-reactive protein were found in this case. Ultrasonography, computed tomography, magnetic resonance imaging, and angiography indicated either mesenteric leiomyoma or leiomyosarcoma, but no definitive preoperative diagnosis could be established. A surgical resection of the tumor revealed a mesenteric Castleman's tumor with small daughter lymphoid tumors of plasma cell type.