Preliminary results from a pilot trial on trastuzumab's mechanism of action against operable breast tumors overexpressing Her2 suggested a role for antibody-dependent cell cytotoxicity (ADCC). To examine factors affecting ADCC intensity and variability, we extended this study to the phenotypic and functional analysis of circulating mononuclear cells in 18 patients. ADCC was induced by trastuzumab therapy in 15 of 18 patients (83%). Inability to develop ADCC in three patients did not depend on inadequate levels of trastuzumab because further increase in its concentration in vitro was ineffective. Rather, susceptibility to develop ADCC was fairly predicted by test with trastuzumab before therapy and was correlated to the number of lymphocytes coexpressing CD16 and CD56. Phenotypic analysis at the end of ADCC evaluating down-regulation of CD16, and up-regulation of CD69 and CD107a, confirmed that natural killer (NK) cells and CD56⁺ T cells were involved in productive engagement of trastuzumab. Also, the killing efficiency of CD16⁺ lymphocytes was influenced by 158 V/F polymorphism of FcγRIII (CD16), whereas variations of CD247 on NK cells were consistent with trends between ADCC before and after therapy. Complete pathologic response was observed in one patient showing ADCC of outstanding intensity, whereas four cases of partial response showed intermediate ADCC; none of the three patients unable to mount ADCC had significant tumor regression. These data indicate that quantity and lytic efficiency of CD16⁺ lymphocytes are major factors for ADCC induction by trastuzumab, and confirm that breast cancer responses to short-term trastuzumab monotherapy may depend on involvement of the ADCC mechanism. [Cancer Res 2007;67(24):11991–9]