Three cases of myeloproliferative disorders in patients with breast cancer are described. The first patient developed acute myeloblastic leukemia 26 years after her initial breast cancer; the second patient developed chronic myelogenous leukemia three years after the diagnosis of breast cancer; the third patient had polycythemia vera for nine years before cancer of the breast was noted. The literature dealing with the association of cancer and myeloproliferative disorders is reviewed. Possible explanations for this association are considered.