Magnetic resonance imaging in a 54-year-old Japanese man showed a huge low-intensity abdominal mass on T1 WI and a high-intensity tumoral structure with low-intensity bundles on T2 WI. The histologic diagnosis was intra-abdominal mesenteric fibromatosis. As the levels of tissue estrogen and progesterone receptors were not elevated (both less than 5 FMOL/mg), tamoxifen treatment was not indicated.