Mortality rates for breast cancer in Japan have been increasing sharply. Earlier detection leading to early treatment is highly desirable. Despite improvement in imaging techniques for breast cancer, many cases with latent microcarcinoma go undetected each year. In Western medicine, diagnostic hormonal therapy with antiestrogens for mastopathy in order to induce latent carcinoma has been used successfully for diagnosis of such difficult cases. Antiestrogens, however, may cause intolerable side effects for some patients, and sometimes their administration must be stopped. Thus excisional biopsy is required for definitive diagnosis of malignancy. In oriental medicine, Keishi-bukuryo-gan has been shown to be effective for mastopathy and is considered quite safe. We examined it as an alternative to antiestrogens. Keishi-bukuryo-gan was administered to 116 patients out of 218 with mastopathy. In response, four were suspected to have breast cancer. All four were definitively diagnosed with breast cancer: two by fine-needle aspiration cytology, and two by excisional biopsy. Keishi-bukuryo-gan proved to be useful in diagnostic hormonal evaluation of mastopathy and diagnosis of breast cancer.