Trastuzumab has changed the natural history of metastatic HER2 positive breast cancer. Some patients remain well and in remission for many years. There is currently no established duration after which trastuzumab in the advanced setting can be safely discontinued. This study aims to evaluate long-term efficacy and cardiac safety of trastuzumab when used as first-line treatment for patients with metastatic HER2 positive breast cancer.We retrospectively identified 215 patients with HER2 positive, locally advanced or metastatic breast cancer who commenced first line trastuzumab-containing therapy for metastatic disease between 2001 and 2010 at The Royal Marsden Hospital.The median progression free survival for all patients was 12 months (95%CI: 10.3-14.6 months); 103 (48%) patients remained in remission beyond one year, 59 (27%) beyond two years and 25 (12%) beyond five years. The median overall survival was 2.6 years (95% confidence interval (CI): 2.2-3.3). The objective response rate (ORR) was 65% with 17 (8%) complete responses and 120 (57%) partial responses. Trastuzumab was well tolerated. Twenty eight (13%) patients recorded any grade of left ventricular dysfunction. There was no significant difference in cardiac toxicity between those patients on less than or more than one year of trastuzumab.Trastuzumab is associated with long-term remissions in a significant proportion of patients with metastatic HER2 positive disease when used in the first-line advanced setting.