A case of nodular histiocytic hyperplasia of the endometrium is described. A 45-year-old Japanese woman was found to have an enlarged uterus during her annual checkup. Hysterectomy and bilateral salpingo-oophorectomy specimens revealed uterine leiomyomas, adenomyosis, and acute salpingitis. A 5-mm, well-demarcated, elevated endometrial nodule (an incidental finding) was present and consisted of round or polygonal histiocytic cells with eccentric nuclei and pale or granular cytoplasm. The nuclei were ovoid, reniform, or crescent-shaped and had fine chromatin and inconspicuous nucleoli, and the cytoplasm contained single or multiple vacuoles. Immunohistochemically, the histiocytic cells were positive for vimentin, CD68, and lysozyme and were negative for cytokeratin, S100 protein, estrogen and progesterone receptors, and CD10. Nodular histiocytic hyperplasia in the endometrium is considered to be a reactive process. Differentiation from neoplasms, including signet-ring cell carcinoma, in curettage specimens is critical to avoid unnecessary surgical resection.