PURPOSE: The purpose of this study was to evaluate the usefulness of sonographic BI-RADS in differentiation between benignity and malignancy of complex breast cysts. MATERIALS AND METHODS: Among 855 female patients who underwent ultrasound-guided core needle biopsy at our hospital from March 2012 through August 2012, 82 pathologically proven complex cysts detected on ultrasonography were included. The BI-RADS classification was made by radiologists prospectively and the imaging characterization of cystic lesions were classified retrospectively as cystic masses with a thick septa or thick wall, mixed solid and cystic masses, or solid masses with eccentric cystic foci. All lesions examined by ultrasonography were classified prospectively according to the BI-RADS lexicon. The pathologic findings were compared with sonographic appearance. We reviewed and compared the radiologic features between malignant lesions and benign lesions. RESULTS: Finally, according to biopsy results, there were 71 benign and 11 (13.4%) malignant lesions. The frequency of malignancy was significantly higher in solid masses with eccentric cystic foci (p value <0.025). The size of sonographic lesions was significantly larger in malignant lesions than in benign lesions (41.82 mm+/-34.36 vs 18.41 mm+/-16.8 respectively, p=0.049). No statistically significant difference was observed in the sonographic findings according to the BI-RADS lexicon, however, statistically significant difference in final assessment categories was observed between benign and malignant lesions (p value <0.001). CONCLUSIONS: The malignancy rate was higher in cystic lesions with predominantly solid masses and classified as suspicious malignancy using the BI-RADS lexicon.