Purpose: Liquid-based cytology have been developed and currently used to overcome the limitation of conventional cytology. This study aims to evaluate the differences between conventional cytology and ThinPrep Ⓡ in the bronchial washing. Materials and Methods: Both conventional cytology and ThinPrep Ⓡ were performed in the 2,551 cases of bronchial washing, of which, 116 cases of histologically diagnosed as lung cancer were evaluated. Results: Among the 116 cases, 23 and 4 cases were negative in conventional cytology and ThinPrep Ⓡ , respectively. The thirty eight cases in conventional cytology and 30 cases in ThinPrep Ⓡ were diagnosed as "atypical." Malignant carcinoma was 54 cases and 82 cases in conventional cytology and ThinPrep Ⓡ , respectively. The positive predictive value, negative predictive value, sensitivity and specificity were 98.94%, 99.06%, 80.17%, and 99.96%, respectively in conventional cytology. Those were 99.12%, 99.83%, 96.55% and 99.96%, respectively in ThinPrep Ⓡ . Conclusion: ThinPrep Ⓡ (liquid-based cytology) revealed higher sensitivity and lower false negative rate than conventional cytology. ThinPrep Ⓡ has many advantages to diagnose the lung cancer from bronchial washing cytology. (J Lung Cancer 2012;11(2):84 88)