Background Few studies have evaluated concomitant pegylated liposomal doxorubicin (PLD) plus trastuzumab as therapy for HER2-overexpressing metastatic breast cancer (MBC). This open-label, prospective, phase II trial assessed the safety and efficacy of this regimen, with cardiac tolerance as the principal focus. Patients and Methods Women with HER2-overexpressing recurrent MBC, baseline left ventricular ejection fraction ≥ 55%, and no history of serious cardiac illness were eligible; preexisting cardiac risk factors, including previous anthracyclines and previous trastuzumab for MBC, were allowed. Patients received weekly trastuzumab and every-3-week PLD until progression, prohibitive toxicity, or patient refusal. Left ventricular ejection fraction was assessed during and after therapy. Grade 3/4 congestive heart failure (CHF) was monitored for premature closure. Results The trial closed after 2.5 years for slow accrual. Twelve patients were enrolled: 7 had received adjuvant anthracyclines; 9 had received previous MBC treatment, of whom 7 had received trastuzumab in combination with chemotherapy. Patients received a mean of 4.8 cycles of PLD; 8 patients experienced stable disease; 4 patients experienced progression. Mean left ventricular ejection fraction levels did not change substantially: 60.4%, 57%, 60.3%, and 56.8% at baseline, after cycle 2, after cycle 4, and after completion of treatment, respectively. No patients experienced grade 4 CHF. One patient discontinued treatment after grade 3 CHF. Three patients experienced grade 2 left ventricular dysfunction, of whom 2 discontinued treatment. Cardiac function improved in all 4 patients after going off study. Other adverse events were generally mild (grade 1/2) and infrequent. Conclusion Pegylated liposomal doxorubicin plus trastuzumab might be an option for heavily pretreated patients with recurrent HER2-overexpressing MBC.