Abstract Purpose The efficacy and tolerability of 2 different schedules of paclitaxel/carboplatin/trastuzumab for HER2-overexpressing metastatic breast cancer (MBC) were evaluated in this parallel multicenter phase II trial. Patients and Methods Patients received every-3-week therapy (n = 43) consisting of a 200 mg/m 2 dose of paclitaxel/carboplatin area under the curve (AUC) of 6 mg/mL per minute and trastuzumab (an initial 8 mg/kg dose and subsequent 6 mg/kg doses) administered every 21 days for 8 cycles or weekly therapy (n = 48) consisting of an 80-mg/m 2 dose of paclitaxel/carboplatin AUC of 2 mg/mL per minute for 3 of 4 weeks, with weekly trastuzumab (an initial 4-mg/kg dose and subsequent 2-mg/kg doses) administered every 4 weeks for 6 cycles. Trastuzumab was continued until disease progression or unacceptable toxicity. HER2 status was confirmed by a central laboratory review. Results The overall response rate (ORR) with every-3-week therapy was 65% (90% confidence interval [CI], 51%–77%), with a median time to disease progression of 9.9 months and median overall survival (OS) time of 2.3 years. The ORR with weekly therapy was 81% (90% CI, 70%–90%), with a median time to disease progression of 13.8 months and a median OS time of 3.2 years. Hematologic and nonhematologic toxicities occurred significantly less frequently with weekly therapy versus every-3-week therapy: grade 3/4 neutropenia (52% vs. 88%); grade 3 thrombocytopenia (4% vs. 30%); and grade 3 neurosensory toxicity (2% vs. 19%), respectively. Conclusions Every-3-week and weekly regimens of paclitaxel/carboplatin/trastuzumab are highly active in women with HER2-overexpressing MBC. However, fewer patients developed severe neutropenia, leukopenia, or thrombocytopenia with the weekly schedule.