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This purpose of this study was to retrospectively stratify the risks of malignancy according to the mammographic characteristics of Japanese women. We studied the mammographic findings of 1267 Japanese women. We characterized malignant phenotypes according to mass shape, margin and mass density, and by shape and distribution of calcified foci, and to obtain possible predictors for malignancies according to age groups. Lobular and irregular mass shape, no circumscribed margin and higher density turned out to be more powerful predictors for malignancy than other radiological factors (< 0.001, respectively). The ratio of the cases detected as a mass in those between 21 and 49 years was lower than that of other age groups. In addition, the presence of calcifications and no mammographic abnormalities were the most powerful predictors for malignancies in the young age groups (P < 0.001, respectively). The peak age of breast cancer is between 40 and 49 years in Japan. In the present study, subtle differences were found in the mammographic results for young and old women, in contrast to those of women in the USA and Europe. The results of this study might enable more accurate prediction of biological behavior of the breast lesions in Japanese women. (Cancer Sci 2012; 103: 472–476)