Malignant struma ovarii is a very rare tumor, consisting of a struma ovarii with malignant change. Only 17 examples with metastases have previously been reported. We present a case which involved both ovaries and produced multiple metastases. Microscopy and ultrastructure were identical to a mixed papillary and follicular carcinoma of the thyroid gland. Characteristic ultrastructural features of malignant struma ovarii are: microvilli on the surface and in the cytoplasmatic vacuoles; oncocytic transformation of cytoplasm of some cells; and presence of electron dense, granular and crystalline material (colloid) in the follicles and vacuoles. The patient's thyroid was normal. We conclude that malignant struma ovarii can only be diagnosed wheb a carcinoma arising in the thyroid gland can be excluded. Ultrastructure examination may provide useful diagnostic features separating malignant struma ovarii from strumal carcinoid and serous adenocarcinoma.