Steroid sulphatases regulate many important physiological processes, including the formation of neurosteroids, some aspects of reproductive function and part of the immune response. Dehydroepiandrosterone sulphatase in macrophages regulates the progression of T0 helper cells to the Th1 phenotype (Daynes et al. 1993, Rook et al. 1994) which secrete cytokines which may be involved in the development of some autoimmune diseases. However, their pivotal role in regulating oestrogen synthesis in endocrine-dependent tumours has been the greatest stimulus in developing potent steroid sulphatase inhibitors. Carlstrom et al. (1984a) made the initial observation that danazol, an isoxazole derivative of 17α-ethinyl testosterone, possessed steroid sulphatase inhibitory properties when it was found that the ratio of dehydroepiandrosterone sulphate (DHEA-S) to unconjugated DHEA increased in women treated with this drug for endometriosis. Subsequent in vitro studies confirmed the ability of danazol to inhibit DHEA sulphatase activity in breast tissues (Carlstrom et al. 1984b).