Testosterone is important for the development of secondary sexual characteristics in female-to-male (FtM) transsexuals, but it may increase breast cancer risk. To date, only one breast cancer case has been reported in the literature in a FtM transsexual after 10 years of testosterone therapy. We describe 2 cases of breast cancers diagnosed in FtM transsexuals who have been treated with supraphysiological doses of testosterone. Our 2 cases demonstrate the unique issues that concern the management of FtM transsexuals with breast cancer and examine possible roles of testosterone in the development of breast cancer.