BACKGROUND: The Bethesda System for Reporting Thyroid Cytopathology (BSRTC) uses six diagnostic categories to standardize communication of thyroid fine-needle aspiration (FNA) interpretations between clinicians and cytopathologists. Since several studies have questioned the diagnostic accuracy of this system, we examined its accuracy in our hospital. METHODS: We calculated the incidences and malignancy rates of each diagnostic category in the BSRTC for 1,730 FNAs that were interpreted by four cytopathologists in Gangnam Severance Hospital between October 1, 2011, and December 31, 2011. RESULTS: The diagnostic incidences of categories I-VI were as follows: 13.3%, 40.6%, 9.1%, 0.4%, 19.3%, and 17.3%, respectively. Similarly, the malignancy rates of these categories were as follows: 35.3%, 5.6%, 69.0%, 50.0%, 98.7%, and 98.9%, respectively. In categories II, V, and VI, there were no statistically significant differences in the ranges of the malignancy rates among the four cytopathologists. However, there were significant differences in the ranges for categories I and III. CONCLUSIONS: Our findings suggest that institutions that use the BSRTC should regularly update their diagnostic criteria. We also propose that institutions issue an annual report of incidences and malignancy rates to help other clinicians improve the case management of patients with thyroid nodules.