A clinicopathologic study of 153 cases of thyroid follicular Hurthle cell tumor (HCT) is presented. Follow-up, available in all cases, ranged from 5 to 26 years (median, 9 years). These neoplasms were divided into three categories on the basis of presence and degree of capsular and vascular invasion, pattern of growth (follicular, trabecular, or solid), nuclear atypia, and necrosis. The categories were benign (90 cases), indeterminate (35 cases), and malignant (28 cases). All the tumors classified histologically as benign or indeterminate behaved in a clinically benign fashion. Of the 28 tumors classified histologically as malignant, 20 had a clinically malignant behavior. At survival analysis, no clinical or pathologic feature among the carcinomas was found to correlate with a fatal outcome. It is concluded that histologic criteria alone can distinguish benign from malignant HCT and that clinical or pathologic feature cannot predict behavior among the malignant tumors.