We compared the anti-estrogen receptors (ER) SP1, 6F11, and 1D5 antibodies in breast carcinoma cases with different ranges of positive cells to evaluate whether this could generate different therapies for patients. We selected 66 cases of breast cancer, each of which was immunostained with the 3 antibodies. 1D5 was less sensitive than SP1 and 6F11, as seen in 26, 20, and 21 negative cases, respectively. Nine cases showed differences in endocrine-therapy indications, of which 8 1D5-negative cases showed low positivity for SP1 and/or 6F11. However, these cases were prevalently G3, progesterone receptor-negative or low-positive, with high Ki-67 and positive HER-2 findings, all biological features associated with endocrine resistance. Finally ER values obtained with these 3 antibodies had no implications for chemotherapy.