Hurthle cell tumors of the thyroid are uncommon neoplasms that generate considerable controversy regarding the correct clinicopathologic criteria for determining which lesions require aggressive surgical management and which lesions require conservative therapy. The neoplasms range from benign tumors that disappear spontaneously to tumors with widespread metastasis. The origin of these tumors is unclear, their clinical course is variable and largely unpredictable, and diagnosis is difficult. Neither atypical architecture nor cytologic features are reliable indicators of malignancy. Surgery is the only effective treatment modality. The authors present two case studies of patients with Hurthle cell carcinoma. They also discuss the natural history and management of this disease.