Leiomyomatosis of lymph nodes is an extremely rare disease. Only a few cases have previously been reported in pelvic lymph nodes. They were related to a benign uterine leiomyoma, a metastasizing uterine leiomyoma, an endometrial adenocarcinoma, and an ovarian endometrioid carcinoma. We report on a case of leiomyomatosis of the mesenteric lymph nodes associated with a duodenal adenocarcinoma with no history of uterine leiomyoma or any gynecological malignancy. The patient, a 56-year-old woman, was found to have an adenocarcinoma of the duodenum. All mesenteric lymph nodes removed showed leiomyomatosis, which was verified by immunohistochemical study showing positive immunostaining for smooth muscle actin, desmin, and vimentin, but negative staining for HMB-45. It is necessary to make a differential diagnosis from other examples of spindle cell proliferation involving lymph nodes such as a hemorrhagic spindle cell tumor with amianthoid fibers (palisade myofibroblastoma), angiomyolipoma, lymphangiomyomatosis, inflammatory pseudotumor, and Kaposi's sarcoma.