Progesterone receptor (PR) function, while essential in normal human breast, is also implicated in breast cancer risk. The two progesterone receptors, PRA and PRB, are co-expressed at equivalent levels in normal breast, but early in carcinogenesis normal levels of PRA:PRB are frequently disrupted, and predominance of one isoform, usually PRA, results. In model systems, PRA and PRB have different activities, and altering the PRA:PRB ratio in cell lines alters PR signaling. The purpose of this study was to determine whether hormonal or reproductive factors contribute to imbalanced PRA:PRB expression in breast tumors and the impact of PRA:PRB imbalance on disease outcome. The relative expression of PRA and PRB proteins was determined by dual immunofluorescence histochemistry in archival breast tumors and associations with clinical and reproductive history assessed. PRA:PRB expression was not influenced by reproductive factors, whereas exogenous hormone use (menopausal hormone treatment, MHT) favored PRB expression (p�<�0.035). The PRA:PRB ratio may be a discriminator of response to endocrine therapy in the TransATAC sample collection, with high PRA:PRB ratio predicting earlier relapse for women on tamoxifen, but not anastrozole (mean lnPRA:PRB ratio; HR (95�% CI) tamoxifen 2.45 (1.20–4.99); p value 0.02; anastrozole 0.80 (0.36–1.78); p value 0.60). The results of this study show that PRA:PRB imbalance in breast cancers is not associated with lifetime endogenous endocrine and reproductive factors, but is associated with MHT use, and that PRA predominance can discriminate those women who will relapse earlier on tamoxifen treatment. These data support a role for imbalanced PRA:PRB expression in breast cancer progression and relative benefit from endocrine treatment.