PURPOSE Pharmacokinetics (PKs) and safety results from phase II/III trials suggest that, if high trastuzumab serum concentrations are reached early during treatment for human epidermal growth factor receptor 2 (HER2)-positive breast cancer, patients will gain clinical benefit, and the synergistic effects of trastuzumab and chemotherapy will be maximized. This phase I/II study evaluated the PKs, efficacy, and safety of a novel, intensive loading regimen of trastuzumab in women with HER2-positive metastatic breast cancer (MBC). PATIENTS AND METHODS An intensive loading regimen of trastuzumab was given (6 mg/kg intravenously on days 1, 8, and 15 followed by 6 mg/kg every 3 weeks from day 22) to women age 18 years or older with HER2-positive MBC who may have received previous surgery, radiotherapy, and/or chemotherapy. Study medication was continued until disease progression or withdrawal occurred. Results All eligible women (N = 72) received at least one dose of trastuzumab. Median estimated trough concentration of trastuzumab at the end of 3 weeks of the intensive loading regimen (total of 18 mg/kg of trastuzumab administered) of cycle 1 was 119 mg/L, which is higher than steady-state trough concentrations with a conventional weekly or every-3-week regimen (64.9 or 47.3 mg/L, respectively). No new or unexpected adverse events or increased cardiotoxicity were reported during the study. In patients with measurable disease (n = 47), response rate was 23.4%. Median time to progression was 7.7 months (in all patients). CONCLUSION An intensive loading regimen of trastuzumab achieved higher-than-steady-state serum concentrations during cycle 1, was well tolerated, and had a good efficacy profile.