In Gunma prefecture, mass screening for both breast and thyroid cancer at the same time was conducted between April 1980 and March 1988 with the cooperation of the Gunma Cancer Society. In this paper, the results of mass screening for breast cancer and further problems are reported and discussed. The total number of subjects during these eight years was 191, 603 on the first screening with physical examination alone. Of these, 18,208 (9.5%) examinees were required to attend the second examination. Breast cancer was detected in 159 cases. The detection rate was 0.08% among total subjects. Also thyroid cancer was detected in 244 (0.13%). The detection rate for breast cancer was approximately twice as high at the initial screening (0.11%) than at the subsequent periodic screening (0.06%). Patients with breast cancer detected by mass screening showed an earlier stage and a better prognosis compared with those of our ordinary outpatients. The 8-year disease-free survival rate in mass screening and in outpatient cases was 94.9% and 83.2%, respectively, but there was no significant difference between the two groups. Interval breast cancer was also found in 19 cases (Tis; 1, Stage I; 8 and Stage II; 10) and 5 patients had nodal metastases. This mass screening program is a useful means for early discovery and early treatment of breast cancer. Hereafter, we should intend to increase the number of initial examinees and women aged 60 and over. Also it is important to make and train physicians for such screening, and to educate women about the practice of breast self-examination.