Central nervous system (CNS) metastases from breast cancer are common and can present as the first or solitary site of disease progression. The CNS has been reported to act as a sanctuary site that denies access to many chemotherapeutic agents. We present here, a series of 10 metastatic breast cancer patients who developed CNS metastases after an initial response to trastuzumab treatment. Forty one patients with metastatic HER2-overexpressing breast cancer, without evidence of CNS involvement prior to the initiation of trastuzumab treatment, were followed during trastuzumab treatment. A neurological evaluation was performed in those patients who developed neurological signs or symptoms during the course of treatment. The clinical course and pattern of CNS involvement in these patients are discussed. Thirty two patients (78%) showed an initial response to trastuzumab treatment. Ten (31%) of the responding patients developed either isolated CNS relapse or concurrent CNS and systemic progression at a median of 43 weeks after the initiation of trastuzumab treatment. Trastuzumab as a single agent was continued following control of brain symptoms in three patients, two showed signs of systemic disease progression at 11 and 15 weeks following the diagnosis of CNS metastases, respectively. In two other patients, trastuzumab in combination with weekly chemotherapy was continued for more than 20 weeks after CNS relapse without evidence of disease progression. The incidence of CNS involvement in our group of patients was higher than expected. With more successful and prolonged systemic anti-tumour effects achieved by novel drug combinations, the risk of developing CNS metastases might be even greater. Evaluation of prophylactic cranial irradiation strategies might be studied for high-risk patients.