The natural history of 27 cases of biologically malignant struma ovarii from a series of 88 cases of histologically malignant or histologically proliferative struma ovarii is described. The extraovarian spread was evident at presentation in 17 patients. The malignant nature of the other 10 tumors became apparent only after they recurred. The tumors measured 5 to 24.5 cm and were more than 50% thyroid tissue in all but 2 cases. The microscopic diagnosis of the thyroid tissue was follicular adenoma in 17 cases (63%), papillary carcinoma in 7 (26%), unremarkable in 2 (7%), and follicular carcinoma in 1 case (4%). Generally, the clinical course was protracted, with long-term survival documented in most patients. Clinical features predictive of biologic malignancy were the presence of adhesions, peritoneal fluid (> or = 1 L), or a serosal rent in the struma ovarii (including cystectomy). In addition, pathologic factors predictive of a poorer prognosis were large size (> or = 10 cm), strumal component more than 80%, and extensive papillary carcinoma, especially with solid areas, necrosis, and > or = 5 mitoses per 10 high-power fields. Follow-up for all patients was 1.5 to 33 years (mean=13.5 yr). On last follow-up 3 patients (11%) had no evidence of disease, 9 (33%) were alive with disease, 5 (19%) died of other causes, and 10 patients (37%) died of the disease. Death from disease occurred 1.5 to 32 years after diagnosis (mean=14 yr). Recurrence was seen as early as 2 months and as late as 29 years after initial surgery (mean=7 yr). Long-term follow-up is indicated in patients with any of the above-mentioned adverse indicators.