Purpose: AS1402 is a humanized immunoglobulin G1 antibody that targets the aberrantly glycosylated antigen MUC1, which is overexpressed in 90% of breast tumors and contributes to estrogen-mediated growth and survival of breast cancer cells in vitro by modulating estrogen receptor (ER) activity. Aromatase inhibitors have been reported to enhance antibody-dependent cell-mediated cytotoxicity elicited by antibodies in vitro. We compared the outcomes of patients with breast cancer treated with letrozole with or without AS1402. Experimental Design: The study population included 110 patients with locally advanced or metastatic hormone receptor-positive breast cancer randomized to receive 2.5 mg letrozole only once daily or with a weekly 9 mg/kg AS1402 infusion. The primary endpoint was overall response rate. Secondary endpoints included progression-free survival, time to progression, and safety. AS1402 exposure and influence of allotypes of FcγRIIIa, FcγRIIa, and MUC1 were evaluated.Results: The study was stopped early because of a trend toward worse response rates and a higher rate of early disease progression in the AS1402 + letrozole arm. Final analysis revealed no significant difference in efficacy between the study arms. Evaluated gene polymorphisms did not define patient subgroups with improved outcomes. Addition of AS1402 to letrozole was associated with manageable toxicity.Conclusions: Because adding AS1402 to letrozole did not improve outcomes compared with letrozole only, blocking ER may be a better strategy for harnessing MUC1 modulation of the ER to a clinical advantage. FcγRIIIa, FcγRIIa, and MUC1 allotype did not predict outcome for patients treated with letrozole with or without AS1402.