An 84-year-old woman underwent total hysterectomy, bilateral salpingo-oophorectomy, and omentectomy 18 years ago because of right ovarian fibrothecoma. After surgery, she was followed up regularly. During follow-up, she presented no androgenic or estrogenic manifestations. She was found to have a pelvic tumor that was suspected to be recurrence of fibrothecoma. Her pelvic tumor was located in the mesentery of the distal ileum. She underwent ileocecal resection to remove the mesenteric tumor. She received no additional therapy. She had an uneventful postoperative course and no recurrence for 1 year after surgery. We re-evaluated the right ovarian tumor collected 18 years ago. The right ovarian tumor was a yellowish white solid tumor the size of an adult head. Fourteen tissue sections were prepared from the right ovarian tumor. Microscopically, theca cell-like cells and collagen-producing fibroblasts were observed in all tissue specimens. No SLCT component was observed in any of the tissue specimens. We diagnosed the patient with fibrothecoma as a result of re-evaluation. Macroscopically, the mesenteric tumor was nodular and well circumscribed, measuring 75 × 65 × 50 mm. The cut surface was yellow. The tumor did not invade into the ileal wall. Microscopically, duct-like structures, which consisted of Sertoli cell-like tall columnar cells, were observed in the diffuse growth of scant cytoplasmic ovoid cells. Additionally, nests of Leydig cell-like cuboidal cells with eosinophilic cytoplasm were observed. The mitotic rate of the tumor was 2 per 10 high-power fields. No heterologous elements were observed. An automatic staining machine (DAKO Envision+ system; DakoCytomation, Glostrup, Denmark) was used for the immunohistochemical procedure. The antibodies used in this study are shown in Table. Positive immunohistochemical expression of steroidogenic factor-1 (SF-1; Fig. a), inhibin-α, cluster of differentiation 56 (CD 56; Fig. c), Wilms tumor 1 (WT-1; Fig. d), AE1/AE3, and vimentin was found in Sertoli cell-like tall columnar cells. Inhibin-α and vimentin were expressed in Leydig cell-like cuboidal cells. Positive expression of SF-1, inhibin-α, CD56, and vimentin was found in ovoid cells. Finally, we examined hotspot mutations in exons 24 and 25 encoding the DICER1 ribonuclease IIIb domain using a direct sequencing method. Before DNA extraction, neoplastic cells accounted for at least 50% of the tissue cell population. DNA was extracted from formalin-fixed, paraffin-embedded tissues. No DICER1 hotspot mutation was detected in this tumor tissue. We therefore diagnosed the patient with primary mesenteric moderately differentiated SLCT without DICER1 hotspot mutation.