PURPOSE: In the absence of head-to-head trial data, network meta-analysis (NMA) was used to compare trastuzumab emtansine (T-DM1) with other approved treatments for previously treated patients with unresectable or metastatic HER2-positive breast cancer (BC). METHODS: Systematic reviews were conducted of published controlled trials of treatments for unresectable or metastatic HER2-positive BC with early relapse (≤ 6 months) following adjuvant therapy or progression after trastuzumab (Tras) + taxane published from January 1998 to January 2018. Random-effects NMA was conducted for overall survival (OS), progression-free survival (PFS), overall response rate (ORR), and safety endpoints. RESULTS: The NMA included regimens from seven randomized controlled trials: T-DM1 and combinations of Tras, capecitabine (Cap), lapatinib (Lap), neratinib, or pertuzumab (Per; unapproved). OS results favored T-DM1 over approved comparators: hazard ratio (HR) (95% credible interval [95% CrI]) vs Cap 0.68 (0.39, 1.10), LapCap 0.76 (0.51, 1.07), TrasCap 0.78 (0.44, 1.19). PFS trends favored T-DM1 over all other treatments: HR (95% CrI) vs Cap 0.38 (0.19, 0.74), LapCap 0.65 (0.40, 1.10), TrasCap 0.62 (0.34, 1.18); ORR with T-DM1 was more favorable than with all approved treatments. In surface under cumulative ranking curve (SUCRA) analysis T-DM1 ranked highest for all efficacy outcomes. Discontinuation due to adverse events was less likely with T-DM1 than with all comparators except neratinib. In general, gastrointestinal side effects were less likely and elevated liver transaminases and thrombocytopenia more likely with T-DM1 than with comparators. CONCLUSIONS: The efficacy and tolerability profiles of T-DM1 are generally favorable compared with other treatments for unresectable or metastatic HER2-positive BC.