Synergism between anti-HER2 monoclonal antibody (trastuzumab) and paclitaxel has been shown in vitro and in vivo. In previous experiences, weekly administration of trastuzumab and paclitaxel has shown significant activity in metastatic breast cancer. In this phase II study, we evaluated the activity and the toxicity of this weekly regimen in anthracycline- and taxane-pretreated patients with HER2-overexpressing metastatic breast cancer. Between November 1999 and July 2001, 25 patients were treated with trastuzumab (4 mg kg(-1) i.v. loading dose followed by 2 mg kg(-1) i.v. week(-1)) and paclitaxel (60-90 mg m(-2) h(-1) i.v. infusion week(-1)). The treatment was planned to continue until disease progression or prohibitive toxicity; in patients with responsive or stable disease, after 6 months of therapy, the decision to stop paclitaxel while continuing weekly trastuzumab was left to the physicians' judgement. At the median follow-up of 19.6 months (range 9.2-38.1), all patients are evaluable for response and toxicity. We obtained four (16%) complete responses (CR), 10 (40%) partial responses (PR), four (16%) stable diseases and seven (28%) disease progressions. The response rate (CR+PR) was 56% (95% CI, 36.5-75.5%). The median duration of response was 10.4 months (range 4.1-24.2+). Median time to progression was 8.6 months (range 2.5-24.2+). The toxicity was mild; five patients experienced fever and chills during the first infusion of trastuzumab (20%); leukopenia grade 2 was recorded in one patient (4%). Two patients (8%) came off study for grade 3 cardiotoxicity (after 9 and 17 weeks of treatment, respectively): both had already received anthracyclines and taxanes. Onycholysis grade 2 was observed in five patients (20%). These results confirm that weekly administration of trastuzumab and paclitaxel is active in anthracycline- and taxane-pretreated metastatic breast cancer patients HER2-overexpressing. Since cardiac disfunctions grade 3 were observed (8%), we recommend that cardiac function should be monitored in these patients.