Androgen deprivation therapy (ADT) is a commonly used treatment for metastatic prostate cancer. A 78-year-old patient with metastatic prostate cancer had transfusion-dependent anemia develop while on ADT. The patient also had hereditary hemorrhagic telangiectasia (HHT), with chronic gastrointestinal blood loss. Blood transfusions were required every 3 weeks for 4 months to keep hemoglobin levels above 8 g/dL, despite discontinuation of ADT. The anemia, which had been well managed with iron therapy before ADT, was worsened by the loss of bone marrow-stimulating testosterone effects. The case highlights testosterone's important role in erythrocyte production. Practitioners should monitor hemoglobin levels in patients undergoing ADT.