Brain metastasis affects one third of patients with HER2-positive breast cancer after treatment with trastuzumab. Surgical resection and radiation therapy are often unsuccessful at accomplishing complete control of metastasis. Lapatinib is presumed to cross the blood-brain barrier, and exhibits clinical activities for treatment of HER2-positive breast cancer. A 43-year-old woman was treated for early breast carcinoma with total mastectomy, axillary lymph-node dissection, and adjuvant chemotherapy with cyclophosphamide plus doxorubicin. After the end of adjuvant trastuzumab therapy, she was diagnosed with panhypopituitarism due to pituitary metastasis. Surgical removal and whole brain radiation therapy were performed, but a portion of viable tumor remained. Only taking lapatinib, the size of the metastatic lesion began to shrink. Trastuzumab may have controlled the micro-metastasis of breast cancer, but it was unable to control its progression to the central nervous system. Lapatinib is a possible option for HER2-positive metastatic breast cancer patients with brain metastasis.