A strumal carcinoid associated with mature cystic teratoma of the ovary in a 59-year-old was investigated immunohistochemically and electron microscopically. Histologically it was composed largely of trabecular and partly of insular carcinoid and individual thyroid follicles. Intensive argyrophilia was shown in both the cells of carcinoid tumor and follicular structure. Thyroglobulin was strongly positive in the follicular lining epithelium and weakly positive in the carcinoid cells adjacent to the follicular area. Immunoreactive cells for somatostatin and prostatic acid phosphatase were strongly detected in the carcinoid area and gradually blended to the follicular epithelium. Methionine-enkephalin, glicentin, and pancreatic polypeptide were focally detected in the carcinoid area. Whereas calcitonin-positive cells were sparsely observed in the follicular area, carcinoembryonic antigen and serotonin were absolutely negative. Electron microscopic findings revealed abundant neurosecretory granules, microfilaments, and colloid-like droplets in the same cells. We suggest that these hybrid cells are the origin of strumal carcinoid.