PURPOSE: Fine needle aspiration cytology has a high sensitivity for the diagnosis of solitary thyroid nodules. Fine needle aspiration cytology (FNA) and frozen section biopsy (FS) have been used to distinguish benign lesions from malignant ones and for deciding the extent of the operative areas. In this study, we correlated data from fine needle aspiration cytology and frozen section biopsy, used in the diagnosis of thyroid nodules, to determine their effectiveness. METHODS: During the period from June 1996 to May 2001, the medical records of 104 patients who underwent a thyroidectomy for a thyroid nodule, at the department of surgery Inha university hospital, were reviewed retrospectively. One hundred and four cases were classified according to whether the FNA cytological diagnosis was inadequate, benign, suspicious, or malignant and the FS diagnosis benign or malignant. RESULTS: Permanent histopathological diagnoeis revealed that 44 cases were benign and 60 cases were malignant. According to the FNA cytologic interpretation, 11 cases were diagnosed as inadequate, 52 cases benign, 20 cases suspicious, and 21 cases malignant. The sensitivity and specificity for FNA alone were 87.8% and 67.3% respectively and for FS 100% and 91.7%. The accuracy of FNA and FS were 76.3% and 96.2% respectively. CONCLUSION: Based on the results, FNA cytology is a useful tool in the initial evaluation of the thyroid nodules and it is as reliable as FS for predicting malignant lesions. In the inadequate, benign and suspicious cases, intraoperative FS is a valuable diagnostic tool to confirm the cytological diagnosis and identify malignancy.