© 2017 John Wiley & Sons LtdObjective: The Bethesda classification was introduced in 2008 to provide standardisation in the evaluation of thyroid fine needle aspiration cytology (FNAC). We compared the diagnostic value of pre-Bethesda and Bethesda classification systems in the differentiation of benign and malignant thyroid nodules. Methods: Medical records of patients who underwent a thyroidectomy between June 2007 and June 2014 were reviewed retrospectively. Nodules evaluated with FNAC before March 2010 were classified as pre-Bethesda (non-diagnostic, benign, indeterminate, suspicious for malignancy and malignant), and those evaluated after March 2010 were considered Bethesda (non-diagnostic, benign, atypia of undetermined significance/follicular lesion of undetermined significance, follicular neoplasia/suspicious for follicular neoplasia, suspicious for malignancy and malignant). Sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV) and accuracy of the two classification systems were calculated. Results: There were 1810 nodules in the pre-Bethesda and 5115 nodules in the Bethesda group. The non-diagnostic rate was significantly higher, and benign and suspicious for malignancy rates were lower in Bethesda compared with the pre-Bethesda group (P<.001 for each). When benign cytology was considered negative, and indeterminate, follicular neoplasia/suspicious for follicular neoplasia, suspicious for malignancy and malignant cytologies were considered positive, results for pre-Bethesda and Bethesda were as follows: sensitivity, 78.9% and 78.5%; specificity, 86.6% and 97.0%; PPV, 42.8% and 72.6%; NPV, 97% and 97.8%; and accuracy, 85.7% and 95.3%, respectively. Conclusions: Among operated nodules, percentages of benign and suspicious for malignancy cytologies decreased, and percentages of non-diagnostic and uncertain cytologies increased with the implementation of Bethesda. The diagnostic value of FNAC seems to have increased with the use of Bethesda classification.