OBJECTIVES: The objective of this study of breast cancer patients enrolled in the EMILIA clinical study was to compare costs of treating patients with either ado-trastuzumab emtansine (T-DM1) or lapatinib plus capecitabine (L+C). METHODS:   All healthcare resource utilization (HCRU) and study drug costs for the EMILIA clinical trial population were included in the estimation of total cost. HCRU costs were estimated using the mean reimbursed amounts from a cohort of breast cancer patients from the Truven Health Marketscan database. Study drug costs were estimated using the unit wholesale acquisition cost for T-DM1 and L+C. Monthly costs were calculated by dividing the total cost by the observed survival time (in months). Total costs were evaluated using the Kaplan Meier Sample Average (KMSA) estimator to account for differences in the length of follow-up and censoring between trial arms. RESULTS: In the EMILIA clinical trial, median overall survival was 30.9 months with T-DM1 (n=495) versus 25.1 months with L+C (n=496) after a median of 19 months follow-up. No differences were observed in the monthly cost per-patient between T-DM1 ($7,151, 95% confidence interval [CI]: $6,753 - $7,550) and L+C ($7,909, 95% CI: $5,095 - $10,723). The KMSA mean total costs were $331,083 (95% CI: $152,238 -  $506,128) and $264,421 (95% CI: $133,143- $412,927) in the T-DM1 and L+C arms, respectively.  Hospitalization costs were lower among the T-DM1 arm ($9,634, 95% CI: $6,394 - $12,873) than the L+C arm ($11,286, 95% CI: $6,216 - $16,357). CONCLUSIONS: Although the mean total costs were higher for T-DMI because of longer survival relative to L+C, the average monthly costs of treating patients with T-DM1 were similar to L+C. This study, along with the 5.8-month survival benefit, supports the potential value of T-DM1 for the treatment of HER2+ metastatic or locally advanced breast cancer.