Retrospective analysis was performed for 56 breast masses with mixed cystic and solid sonographic patterns encountered in the last 5 years at Keio University Hospital, with the objective of elucidating their significance. The lesions were classified into two major groups: tumor with cyst (TWC--solid mass with cystic component) and intracystic tumor (ICT--cystic mass with solid component). According to the pathological diagnosis, 57% were benign and 43% were malignant. Fifty eight percent of malignant lesions were shown to have a TWC pattern and 42% an ICT pattern. Among benign lesions, 66% were shown to be TWC and 34% ICT. In malignant tumors, 96% of the cases showed irregular shape and heterogeneous internal content. Even in benign lesions, a high incidence of irregular shape and rough border was found, although the predominant internal echo texture was homogeneous, and three-dimensional (3-D) contour was not clear. In conclusion, we observed that in the TWC group, the lack of a defined 3-D contour is the most important criterion, and tumors with multiples cysts tend to be benign.