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BACKGROUND: The effectiveness of mammographic screening in women aged over 50 years has been confirmed in the United States and Europe, but its effectiveness in women aged from 40 to 49 years remains controversial. The optimum age for effective screening of subjects for breast cancer by mammography in Japan was studied based on the results of mammographic screening. METHOD: The benefit of breast cancer screening in women was examined by stratifying the results of mammographic screening in a Tokushima trial on the basis of age: under 50 years and 50 years or older. The results of conventional screening by physical examination alone, which we performed in a Zentsuji trial, were used as the control. RESULTS: The examinees numbered 13,982 and 18,619 in mammographic screening and screening by physical examination, respectively. Breast cancer was detected in 43 and 22 patients, respectively. The detection rate of breast cancer was 0.31% by mammographic screening, which is about 3 times higher than that (0.12%) by screening using physical examination. Mammographic screening thus showed significantly higher sensitivity (93.5% vs 73.3%, p = 0.015). The proportion of stage I cancer and the absence of nodal involvement were 67.4% and 79.1% by mammographic screening, compared with 31.8% and 59% by physical examination. Our results obtained with mammographic screening were equal to or higher than the results obtained in the United States and Europe. The clinical stage of the breast cancers detected by mammographic screening in the subjects aged under 50 years was stage 0 (DCIS) in one case and stage I in 10 cases, while the group aged 50 years or older showed stage 0 in 11 cases and stage I in 19 cases. There were three cases of false-negative; two false-negative cases were aged under 50 year, while one case was aged 50 years or older. The detection rates of cancer in the group under 50 years and that of 50 years or more were 0.19% and 0.39% by mammograpic screening and 0.09% and 0.15% by the physical examination. The sensitivities in the group under 50 years and that of 50 years or more were 84.6% and 97.0% by mammographic screening and 72.7% and 73.7% by physical examination, showing no significant difference. In the results of mammograms by Wolfe's classification with respect to the age groups, the proportion of DY (dense breast) pattern decreased significantly from 3.5% to 0.2% in women of 50 years or more and from 16.6% to 2.4% in those under 50 years when the values were compared between the period from 1992 to 1995 and the period from 1998 to 1999, respectively (p < 0.0001). CONCLUSION: The above findings suggested the possible effectiveness of mammographic screening not only in women aged 50 years or more but also in those aged under 50 years, in Japan. Therefore, introduction of mammography should be considered at an early date, even for women aged from 40 to 49 years.