Clinical therapies for breast cancer are guided by estrogen receptor (ER) status determined by immunohistochemical analysis. A previous retrospective study comparing the recently generated rabbit SP1 monoclonal antibody (MAb) with the conventionally used mouse 1D5 MAb reported that 8% of breast carcinomas were SP1+/1D5- (correlating with good outcomes), and 2% were SP1-/1D5+ (correlating with poorer outcomes). This study on mostly previously frozen tissue implied that 1D5 fails to identify some women who may benefit from endocrine therapy. The current prospective study compared SP1 and 1D5 immunostaining on routinely processed consecutive cases of breast carcinoma. ER was classified using the same positive threshold used in the prior study (<1% negative; > or = 1% positive). Of 508 carcinomas, 2 were SP1+/1D5-, and none were SP1-/1D5+. Although SP1 is our preferred antibody, with more intense nuclear staining, both MAbs give similar results in tissue from routine clinical samples with discrepant results in fewer than 0.5% of cases.