Our phase IIa study characterized the safety and efficacy of two human epidermal growth factor receptor 2 (HER2) -targeted agents, trastuzumab emtansine (T-DM1) and pertuzumab, in patients with HER2-positive metastatic breast cancer (MBC). Patients with HER2-positive locally advanced breast cancer or MBC were treated with 3.6 mg/kg T-DM1 plus pertuzumab (840-mg loading dose, then 420 mg subsequently) once every 3 weeks. The primary efficacy end point was investigator-assessed objective response rate (ORR). Sixty-four patients (43 patients in the second-line or greater setting advanced MBC; 21 patients in the first-line setting first-line MBC) were enrolled. Patients with advanced MBC had received trastuzumab and a median of six prior nonhormonal treatments for MBC; 86% of first-line MBC patients had received trastuzumab in the (neo)adjuvant setting. The ORR was 41% overall, 33% in patients with advanced MBC, and 57% in first-line patients. Median progression-free survival was 6.6, 5.5, and 7.7 months, respectively. The most common adverse events were fatigue (61%), nausea (50%), and diarrhea (39%). The most frequent grade ≥ 3 adverse events were thrombocytopenia (13%), fatigue (11%), and liver enzyme elevations (increased ALT: 9%; increased AST: 9%). One patient had left ventricular ejection fraction of less than 40% after study drug discontinuation. Exploratory biomarker analyses demonstrated that patients with above-median tumor HER2 mRNA levels had a numerically higher ORR than patients with below-median levels (44% v 33%, respectively). T-DM1 and pertuzumab can be combined at full doses with no unexpected toxicities. The preliminary efficacy in patients in the first-line and advanced MBC settings warrants further investigation.