The cytologic and histologic slides on all patients with a diagnosis of Hürthle cell tumor at Mount Sinai Hospital during the last 12 years were reviewed. There were 67 Hürthle cell tumors of which 15 (22%) were malignant. Four carcinomas (27%) occurred in a background of thyroiditis. Forty-three patients with Hürthle cell tumors had undergone preoperative fine-needle aspiration, of which 31 had satisfactory aspirates. For Hürthle cell neoplasia, fine-needle aspiration cytology had a sensitivity of 83.8% (26 of 31) and positive predictive value of 93% (26 of 28), provided that the aspirate was of adequate cellularity. All 3 cases predicted as Hürthle cell carcinoma on the basis of cellular and nuclear atypia were correctly predicted, but 3 carcinomas composed of bland cells were incorrectly predicted as adenomas. We conclude that aspiration cytology can differentiate nonneoplastic from neoplastic Hürthle cell lesions with high accuracy, but that the differentiation between benign and malignant lesions is less reliable.