A total of 409 postmenopausal patients with advanced metastatic breast cancer were randomized to receive either formestane (Lentaron) 250 mg every 2 weeks by intramuscular injection, or tamoxifen 30 mg/day orally. Treatment continued until tumor progression. The groups were well matched for pretreatment characteristics including age, performance status, hormone receptor status (patients with known negative receptor status of their primary tumor were excluded), site and extent of metastases, disease-free interval, and previous primary and adjuvant therapy. Patients were assessed for antitumor efficacy at 3-monthly intervals using UICC criteria. Of the 348 patients evaluable for response, 33% had an objective response to formestane (14 complete and 43 partial responses), while 37% had an objective response to tamoxifen (10 complete and 54 partial responses). Median duration of response was 15 months for formestane and 20 months for tamoxifen; survival was 35 and 38 months respectively. There were no statistically significant differences between the treatments for all these variables, but time to disease progression and time to treatment failure significantly favoured tamoxifen. Systemic tolerability was excellent for both treatments. Local side effects due to intramuscular injection of formestane were mild and transient. In this comparative trial of first-line therapy for advanced breast cancer, formestane gave results comparable to tamoxifen for both efficacy and tolerability. We conclude that formestane is an effective and well tolerated addition to the therapeutic options available for the treatment of postmenopausal women with advanced breast cancer.