Metastatic breast cancer is considered an incurable disease. Targeted treatments against the human epidermal growth factor receptor 2 (HER2), however, significantly improve survival in patients with metastatic HER2-positive breast cancer. Some patients may respond with prolonged complete remission. Evidence on safety of long-term trastuzumab and risk of relapse after trastuzumab cessation is limited. We present a case of an 81-year-old patient with HER2-amplified metastatic breast cancer (MBC) in the liver. Following taxane-based chemotherapy in combination with trastuzumab after local treatment resulted in a complete radiological remission after 21 months of trastuzumab maintenance therapy. The patient remains in complete remission 6 years later and continues to receive trastuzumab as maintenance therapy. Prolonged remission in cases with complete response under trastuzumab-based regimens for metastatic HER2-positive breast cancer can be observed in some patients. Reviewing the few available cases published in the literature, these patients share some common characteristics: hormone receptor negative disease and metastases to the liver. There is no evidence that trastuzumab maintenance treatment can be safely interrupted after a certain time period.