Purpose: In breast sonography, “multiple cystic lesions” are defined as numerous cysts existing in clustered formations in segmental or local regions. These lesions are usually caused by mastopathy. In Japan, however, no reports based on these facts have been published. We thus investigated the frequencies, the US findings, the pathology of multiple cystic lesions, and determined their make-up. Subjects and Methods: Thirty cases showing multiple cystic lesions in breast sonographic examinations performed in our hospital (total 6,504 cases) from January 1, 2005 through January 31, 2009 were charted, and we investigated them retrospectively with respect to frequency of malignancies, pathological findings, sonographic findings, and age of patient. Results: The frequency of multiple cystic lesions was 0.46% (30 cases in 6,504 patients). Eight (0.12%) cases were diagnosed as malignancy pathologically (invasive carcinoma, 3 cases; ductal carcinoma in situ, 5 cases). The remaining 22 cases were all diagnosed as benign clinically and/or pathologically. We checked to determine coexistence of low echo and its features. Six cases showed no low echo; 24 cases did. All of the malignant tumors showed low echo; on the other hand, 6 cases showing no low echo were entirely benign. The two groups did not differ in frequency of malignancy. (P=0.15) (group 1: including the group aged of 50 years and above; group 2; subjects under the age of 50). Conclusion: Although low frequencies of multiple cystic lesions were found, these findings included malignant tumors. This points out the importance of distinguishing between multiple cystic lesions and simple cysts.