목적: The Bethesda System for reporting thyroid cytopathology introduced the atypia of undetermined significance (AUS) category, but did not provide adequate guidance for the appropriate use of this diagnosis. This study is designed to determine the cilinical predictors of malignancy in the AUS category. 방법:A retrospective analysis was done on sixty-two patients who underwent thyroid surgery from January 2010 to December 2013, following a diagnosis of AUS from preoperative thyroid FNA. We investigated the age, gender, maximum size and site of the nodules, ultrasonographic findings, cytological features, BRAF gene mutation, surgical method, number of AUS on repeated FNA, and final pathologic results. 결과:Fourty-one out of sixty-two patients underwent total thyroidectomy and the rest had lobectomy. The final pathologic results were fourty-one malignancies and twenty-one benign diseases. Nodules less then 1.5cm, ultrasonographic findings suggestive of malignancy were risk factors for malignancy on univariated analysis. Multivariated analysis showed that nodules less than 1.5cm, ultrasonographic findings suggestive of malignancy and more than 2 results of atypia from prepeated FNAs were significant risk factors for malignancy. 결론:From the results of our study, we recommend surgery or close observation with follow-up examination such as ultrasound-guided needle biopsy for thyroid AUS patients who has high risk factors for malignancy.