Breast cancer is the commonest cancer in women and, despite continuing advances in treatment, each year world-wide an increasing number die from the disease: in Japan the incidence has been increasing by an alarming 10% per annum. In the early stages of the disease, 30-40% of patients respond to hormonal or anti-hormonal therapy. One way to deprive hormone-dependent cancer of its primary mitogens, oestrogens, is to prevent their synthesis-preferably by inhibition of aromatase, the ultimate and biochemically unique enzyme that converts androgens such as testosterone to mitogenic oestradiol.