This article considers the establishment of the category of "hormone-dependent cancers," identified around the middle of the twentieth century as cancers sustained by particular hormones. A comparison of hormonal treatments for prostate cancer and those for breast cancer reveals that the genesis of "hormone-dependent cancer" as a biomedical category relied upon assumptions that cast androgens and estrogens as opposing ends of a gendered hormonal binary of health and disease. In the 1930s, cancer researchers claimed "female sex hormones" (estrogens) exacerbated breast cancer and "male sex hormones" (androgens) prevented it. In the early 1940s, Dr. Charles Huggins applied the opposite logic to the treatment of human prostate cancer, which he determined to be "hormone-dependent." As "hormone dependency" was also recognized in human breast cancer over the subsequent decades, estrogen claimed a prominent place in discussions of breast cancer's causation, diagnosis, and treatment. This close association between estrogen and breast cancer contributed to reinterpretations of both biomedical categories.