Preliminary results of a phase II study of gemcitabine plus trastuzumab in previously treated (up to 3 previous regimens) metastatic breast cancer patients are presented. Patients had histologically confirmed metastatic breast cancer, with 2+ or 3+ tumor HER2 expression. Treatment consisted of gemcitabine 1200 mg/m2 over 30 minutes intravenously on days 1 and 8 every 21 days, and trastuzumab 4 mg/kg over 90 minutes, followed by 2 mg/kg infused over 30 minutes weekly. Treatment was continued until disease progression or unacceptable toxicity occurred. Preliminary results are available on the first 38 patients enrolled. Median patient age was 53 years, 53% had estrogen receptor/progesterone receptor-positive disease, and HER2 staining was 2+ in 39% and 3+ in 61% of patients. There was a median of 3 previously administered (including adjuvant) chemotherapy regimens, and a median of 4.5 treatment cycles per patient has been administered so far. Twelve patients (32%) have had an objective partial response, with a median response duration of 8.6 months. Median time to disease progression is 6.7 months to date, with a median overall survival of 10.2 months. No unexpected toxicities or grade 4 nonhematologic toxicities have been observed; 2 patients developed grade 4 neutropenia and 1 patient had febrile neutropenia. Thus, gemcitabine/ trastuzumab resulted in an encouraging 32% response rate, given the heavily pretreated patient population. Tolerability was good overall, with no unexpected side effects observed.