BACKGROUND Neuroendocrine carcinoma of the non-small cell type of the ovary is a rare aggressive tumor, interestingly associated with either a surface epithelial tumor or teratoma. METHODS A 71-year-old woman presented with a pelvic mass and underwent a total abdominal hysterectomy with a bilateral salpingo-oophorectomy. Pathology examination showed a 6.5 cm in greatest dimension ovarian tumor composed of neuroendocrine carcinoma of the non-small cell type and serous carcinoma. Immunohistochemical studies including keratin 7, WT-1, and neuroendocrine markers demonstrated differences in the two components. Microsatellite instability (MSI) analysis using five polymorphic markers also showed a different pattern in the two components. CONCLUSIONS This is the first report of an ovarian neuroendocrine carcinoma, non-small cell type, associated with a serous carcinoma. Immunohistochemistry and MSI are very helpful in making a definite diagnosis.