During a 25-year period (1959 through 1983), 54 patients with Hürthle cell tumors were treated and monitored at the University of Chicago Medical Center. Thirty percent were men and 70% were women; mean age at diagnosis was 46.7 +/- 13.2 years (range: 19 to 69 years). Tumors were grouped into three categories at the time of initial diagnosis: group 1, grossly malignant (four patients, or 7.5%); group 2, intermediate (partial capsular and/or subcapsular vascular invasion) (10 patients, or 18.5%); and group 3, benign appearing (40 patients, or 74%). Twenty-one (39%) of the patients had a history of low-dose, external radiation to the head and neck in childhood (including three of four grossly malignant lesions). A separate non-Hürthle cell thyroid carcinoma was found within the thyroid gland in 22 (50%) of the patients--79% were papillary and 21% were follicular carcinomas. In half of these, there was a history of childhood irradiation. During a mean follow-up period of 8.4 years (range, 22 days to 35 years), three additional Hürthle cell tumors were recognized as malignant after metastases were discovered--two were originally classified as intermediate lesions and one was in the benign-appearing group. Thus, seven of 54 of our patients (13%) had Hürthle cell carcinomas. One of the seven patients died of widespread metastases after 35 years, and the other six are currently free of disease. We believe that therapy of these lesions should be individualized. Total thyroid ablation (surgical procedure followed by radioiodine therapy) is appropriate for frankly malignant Hürthle cell cancers, for all Hürthle cell tumors occurring in patients who received low-dose childhood irradiation, for associated papillary or follicular carcinomas, and in those patients who exhibit partial capsular or subcapsular vascular invasion. On the other hand, single, well-encapsulated, benign-appearing Hürthle cell tumors may be treated by lobectomy and careful follow-up, since the chance that they will later exhibit malignant behavior is low (2.5% in our series and 1.5% among patients described in the recent literature).