Strumal carcinoid of the ovary is a type of germ-cell tumor characterized by an intimate mixture of thyroid tissue and carcinoid. Fifty patients with this type of tumor ranged in age from 21 to 77 years. The tumors measured up to 26 cm in diameter and were always unilateral. In 10% of the cases, the contralateral ovary contained another type of neoplasm, usually a dermoid cyst. Three-fifths of the strumal carcinoids arose in dermoid cysts or in mature solid teratomas. Birefringent calcium oxalate monohydrate crystals were commonly identified in the thyroid colloid, and argentaffin granules, in the carcinoid cells, thus substantiating the identity of the neoplastic components. Although 31% of the tumors were accompanied by focal stromal luteinization, only 8% of the cases exhibited clinical signs of steroid hormone production (endometrial hyperplasia, hirsutism, or virilism). In 8% of the cases, there was evidence suggesting functioning of the thyroid component, but no patient had the carcinoid syndrome. Only one woman died of tumour; five others died of unrelated causes. The remaining patients remained alive and well; 23, for at least five years, and 15, for at least ten years postoperatively. Although the carcinoid component of the strumal carcinoid has been considered a malignant transformation of struma ovarii, it is almost always benign, and treatment with a simple oophorectomy or salpingo-oophorectomy is effective.