Considerable controversy exists regarding the ability to predict the biologic behavior of Hurthle cell tumors. Some have found the clinicopathologic criteria used to differentiate benign from malignant lesions to be unreliable and have advocated total thyroidectomy for all Hurthle cell neoplasms. From January 1980 to December 1995, 39 patients had surgery for Hurthle cell tumors of the thyroid. The surgical pathologic findings were reviewed by an experienced pathologist (JP). Eight patients had histologic findings of capsular or vascular invasion consistent with carcinoma and had total thyroidectomy. Four of these patients had postoperative evidence of residual disease and were treated by radiation ablation. No evidence of invasion was found in 31 patients diagnosed with Hurthle cell adenoma. Twenty-three of these patients had unilateral lobectomy; total thyroidectomy was done in the remaining 8 patients, 5 of whom were found to have an associated papillary carcinoma at the time of operation. There were no operative deaths or significant morbidity. Twenty-two adenomas (71%) were found in females, whereas males had malignant tumors in 6 of 8 cases (P = 0.025). The mean age of adenoma patients is 54.1 years, and that of the carcinoma patients is 55.8 years. Mean size of benign tumors was 2.8 cm and of malignant tumors 4.1 cm (P = 0.04). Four of seven (57%) carcinomas were larger than 4 cm as compared with 6 of 30 (20%) adenomas (P = 0.069). Follow-up has ranged from 1 month to 15 years, with a mean of 3.2 years. There have been no deaths, and no patients with Hurthle cell adenoma have had evidence of recurrence or metastases during follow-up. Our data suggest that carcinoma patients tend to be male and tumor size is larger. An association was found when trying to predict malignancy by using 4 cm as a threshold size. We conclude that pathologic evidence of capsular or angioinvasion can accurately differentiate benign from malignant tumors. Unilateral thyroid lobectomy is adequate therapy for the treatment of Hurthle cell adenoma, with total thyroidectomy reserved for those patients with histologically proven carcinoma.