A 38-year-old woman with cancer of the left breast underwent a modified radical mastectomy with lymph node dissection. Twenty-one months later, massive liver metastases and pleural carcinomatosis occurred. The liver metastases responded completely to chemotherapy with trastuzumab combined with docetaxel, but the pleural carcinomatosis was refractory to the therapy. Fluorescence in situ hybridization showed that both the primary tumor and the metastatic tumors of the lymph nodes were composed of HER2 amplification-positive and HER2 amplification-negative cancer cells. This analysis also detected a single cell with HER2 amplification in the pleural effusion that was taken at the completion of the chemotherapy, but four follow-up tests showed no amplified cells. It is speculated that in the liver metastases, the trastuzumab was cytotoxic to both HER2-amplified and nonamplified cancer cells and may have acted through its antiangiogenic effect. However, in the pleural effusion, the effect of trastuzumab was more specific to HER2-amplified cells and caused outgrowth of cancer cells lacking expression of HER2 receptors.