Trastuzumab mediates the lysis of HER2-expressing breast cancer cell lines by interleukin-2 (IL-2) primed natural killer (NK) cells. We hypothesized that IL-2 would augment the anti-tumor effects of trastuzumab in MBC in patients who had progressed on or within 12 months of receiving a trastuzumab-containing regimen. Secondary objectives were to measure antibody-directed cellular cytotoxicity (ADCC) against HER2 over-expressing target cells, and to measure serum cytokines. Patients received trastuzumab (4 mg/kg intravenously (IV)) every 2 weeks in combination with daily low-dose IL-2 (1 million IU/m² subcutaneously (SC)) and pulsed intermediate-dose IL-2 (12 million IU/m² SC). Samples were analyzed for NK cell expansion and ADCC against a HER2-positive breast cancer cell line. In addition, interferon-gamma (IFN-γ), mRNA expression in peripheral blood mononuclear cells (PBMC) and the following serum cytokines were measured: IFN-γ, monokine-induced by IFN-γ (MIG), and interferon-inducible protein ten (IP-10). The median number of treatment cycles was four (range 1–23) and the treatment was well tolerated. There were no objective responses. NK cells were not expanded and ADCC was not enhanced. Eight (62%) patients had a twofold or higher increase in mRNA transcript for IFN-γ, two (15%) patients had elevated serum levels of IFN-γ and 12 (92%) had increases angiogenic MIG and IP-10. In trastuzumab-refractory patients adding IL-2 did not produce responses and did not result in NK cell expansion. However, these patients had the ability to respond to IL-2 as evidenced by increases in IFN-γ transcripts and chemokines. The lack of NK cell expansion may explain the absence of clinical benefit.