Breast cancer is the most common cancer in women and affects 1.38 million women worldwide per year. Antiestrogens such as tamoxifen, a selective estrogen receptor (ER) modulator, are widely used in clinics to treat ER-positive breast tumors. However, remissions of breast cancer are often followed by resistance to tamoxifen and disease relapse. Despite the increasing understanding of the resistance mechanisms, effective regimens for treating tamoxifen-resistant breast cancer are limited. Antrodia cinnamomea is a traditional medicinal mushroom native only to Taiwan. In this study, we aimed to examine in vitro effect of antrodia cinnamomea in the tamoxifen-resistant cancer.