In a phase II study, 91 women with advanced breast cancer received formestane (Lentaron) 500 mg intramuscularly every 2 weeks for 6 weeks, and then 250 mg every 2 weeks thereafter; all were either postmenopausal or surgically oophorectomized, and most had received previous hormonal treatment and/or chemotherapy. Complete or partial response was seen in 23%, disease stabilization in 29%. Disease-free interval and site of metastatic lesions did not significantly affect response. Patients who had previously responded to tamoxifen showed the best results with formestane. Side effects were generally mild and transient; only 3 patients had to discontinue treatment. Serum estradiol and estrone levels fell significantly after 2 weeks' treatment, and remained suppressed; there were no significant changes in serum cortisol, blood parameters, and liver and kidney function. Formestane thus appears to be an effective, specific and well-tolerated aromatase inhibitor for the treatment of advanced postmenopausal breast cancer.