Before anti-estrogens such as tamoxifen were developed to treat estrogen receptor (ER)-positive breast cancer, high-dose estrogen therapies were used. This seems counterintuitive since anti-estrogens block ER function, while estrogens increase ER function, but these therapies are effective to similar extents for the treatment of metastatic ER+ breast cancer. Estrogen therapies are most effective against cancers that develop resistance to anti-estrogens, likely because such cancers have adapted to grow without ER function, and restoring ER function (with estrogen) is damaging to the cancer cells. In some patients with ER+ breast cancer that becomes resistant to anti-estrogens, treatment with the estrogen 17B-estradiol induces tumor response. Furthermore, when 17B-estradiol-sensitive tumors eventually become resistant to 17B-estradiol, switching back to anti-estrogen therapy is often effective. These observations suggest that cancers can alternate between anti-estrogen-sensitive and 17B-estradiol-sensitive states. The investigators hypothesize that treatment with alternating 17B-estradiol / anti-estrogen therapies on a defined 8-week / 16-week schedule will more effectively prevent cancer growth than continuous treatment with either type of therapy in patients with metastatic anti-estrogen-resistant ER+ breast cancer.