This retrospective cohort study of 12,500 women with breast cancer, treated in the community, compared the incidence of congestive heart failure in those who received trastuzumab-containing adjuvant chemotherapy regimens with those who did not. There was a four-fold increase in the risk of heart failure in women who received trastuzumab alone and a seven-fold increase in those who received anthracycline plus trastuzumab.