One hundred and twelve post menopausal or post oophorectomy women with advanced breast cancer (BC) who had all previously had aminoglutethimide (AG) were treated with the potent aromatase inhibitor 4-hydroxy androstenedione (4-OHA). Twenty three women (21%) had a partial response to 4-OHA while another twenty five patients (22%) had stabilization of previously progressing disease. Patients responded to 4-OHA both after previously responding to then relapsing on, and after failing to respond to aminoglutethimide. Toxicity was minimal. This study shows that potent aromatase inhibition with 4-OHA is effective in women with advanced BC who have already been treated with a less potent aromatase inhibitor, and suggests that relative changes in oestrogen levels may be more important than absolute levels.