This study investigated the therapeutic effect of single-agent i.v. weekly Navelbine (vinorelbine), a semisynthetic vinca alkaloid, in women who had received no prior treatment for locally advanced or metastatic breast cancer. Of 68 patients entered into the study, 63 were adequate inclusions, assessable for toxicity and response by WHO criteria; the 5 patients who were not evaluated were excluded from analysis because they were found not to meet the eligibility criteria of the study. Navelbine was given as a weekly 30 mg/m2 short i.v. (20 minutes) infusion; treatment was continued until disease progression. The overall response rate was 44% (complete response 8%, partial response 36%). The response rate according to target was lymph nodes, 62.9%; liver, 50.0%; lung, 50.0%; skin, 37.5%; and primary tumor, 30.8%. The median duration of response was 17.9 weeks (range: 7-52 weeks). The median time to treatment failure was 12.9 weeks, and the median survival was 50.3 weeks. The 63 eligible patients received 501 cycles. The mean dose intensity was 76%. At least one episode of WHO grade 3/4 granulocytopenia was seen in 46% of the patients (13.6% of cycles). Significant nausea/vomiting was seen in only 5% of patients, corresponding to 1% of cycles. Only 5% of patients developed WHO grade 3-4 constipation and grade 3 peripheral neuropathy was observed in 1.6% of patients. Alopecia was rare (6.3% of patients), and other side effects were uncommon. This study confirms that Navelbine has major single-agent antitumor activity as frontline therapy in advanced breast cancer. Given its excellent tolerance profile and low morbidity, it should be recommended for inclusion in first-line combination chemotherapy regimens.