Breast cancer is often hormone responsive, since growth or regression of tumors can often be modulated by appropriate endocrine manipulations. Estrogen and progesterone appear to be major hormones involved in regulation of breast tumor growth. It has been recently argued that a more accurate marker of hormonal responsiveness might result if an end product of an intact estrogen response system were measured instead of the initial hormone binding step. Progesterone receptor (PgR) has been investigated in this regard since it can be readily measured in human breast tumors and there is clear evidence in experimental breast tumor model systems that PgR is under acute estrogen control. PgR is rarely found in ER- metastatic breast tumors but is present in approximately 59% of ER+ metastatic tumors, especially in those tumors with high levels of ER. Preliminary clinical correlation of ER, PgR and response to endocrine therapy is encouraging. The response rate is significantly higher if the tumor contains both ER and PgR than if the tumor contains ER alone.