Aromatase inhibitors are a useful therapeutic option in the management of endocrine-dependent advanced breast cancer. Formestane (Lentaron) is the first irreversible aromatase inhibitor to be extensively investigated. In a phase II study to determine the effects of formestane on serum estradiol and urinary 17-hydroxycorticosteroid (17-OHCS) levels and to evaluate its clinical activity, 72 postmenopausal patients with advanced breast cancer were given formestane 250 mg intramuscularly every 2 weeks. Of 66 patients fully evaluable, 56 were estrogen receptor (ER) positive, and 43 had a disease-free interval > or = 2 years. Metastases were assessable in soft tissue (53%), bone (53%) and viscera (47%); 34 patients had 1, 32 had > or = 2 metastatic lesions. Serum estradiol levels fell significantly (p < 0.01) after 2 weeks and remained unchanged thereafter, whereas urinary 17-OHCS levels did not change during treatment. Objective responses were obtained in 19 patients (29%), of whom 8 had complete response. In relation to disease sites, similar responses were obtained in soft tissue (33%) and viscera (30%), whereas response in bone was 18%. The overall tolerability of formestane was satisfactory, and only two patients complained of local side effects. We conclude that formestane is an effective aromatase inhibitor in postmenopausal patients with hormone-dependent breast cancer, and does not interfere with adrenal steroidogenesis.