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Thyroid nodules that are read on cytology as follicular or Hürthle cell neoplasms (FN and HN, respectively) and indeterminate for malignancy require surgery to differentiate benign from malignant nodules. We analyzed FN and HN with indeterminate cytology to determine if there were differences in the rate and types of thyroid malignancy and if the rate of thyroid malignancy was influenced by age or sex.We analyzed 463 nodules with an indeterminate cytological diagnosis of FN and 140 nodules with an indeterminate cytological diagnosis of HN. The histopathological diagnosis after thyroidectomy was the method for establishing the diagnosis and type of malignancy.For the entire series of 603 patients there were 106 (17.6%) with thyroid cancer; 80 of these had a cytology reading of FN and 26 had HN. Extrathyroidal invasion in the grouped HN and FN patients who had papillary thyroid carcinoma (PTC) was more common in females than in males (62% vs. 25 %, p0.05). The rate of thyroid cancer was similar in FN (17.3%) and HN (18.6%). The rate of Hürthle cell thyroid cancer was significantly higher in HN than in FN (5.0% vs. 0.7%, p0.01) and the rate of the oncocytic variant of PTC was also significantly greater in HN compared to FN nodules (23.1% vs. 1.7%, p0.05). The rate of follicular thyroid carcinoma was almost identical in patients with HN and FN (19.2% vs. 18.8 %).There is little difference in the rate of malignancy between thyroid nodules with a cytological reading of FN indeterminate for malignancy and HN indeterminate for malignancy but there is a difference in the types of thyroid cancers in these groups. Hürthle cell thyroid cancer and the oncocytic variant of PTC is more common in nodules with an HN indeterminate for malignancy cytology reading than in nodules with a FN indeterminate for malignancy cytology reading. Since Hürthle cell thyroid cancer and the oncocytic variant of PTC are more aggressive than other thyroid cancers, it is likely that patients with an HN indeterminate for malignancy cytology will, as a group, have more aggressive thyroid cancers than those with an FN indeterminate for malignancy cytology.