The relative merits of the steroidal anti-androgen, cyproterone acetate, and the non-steroidal anti-androgens flutamide and nilutamide, are reviewed. It is concluded that pure anti-androgens offer some advantages over cyproterone acetate but that they each have some features which merit improvement. A new compound which was a pure anti-androgen, peripherally selective, well tolerated with a long half-life would have significant advantages. Preclinical studies in rats and dogs show that Casodex (ICI 176,334) is a potent pure anti-androgen which is well tolerated and has a long half-life. Casodex induces marked regression of the prostate yet fails to cause the substantial elevation in serum LH and testosterone seen with flutamide and nilutamide; it is thus peripherally selective. Casodex is as effective as surgical or medical castration with Zoladex in limiting the growth of the transplantable androgen-responsive Dunning rat prostate tumour. Such a profile makes Casodex a strong candidate as the future anti-androgen of choice for the treatment of prostate cancer and benign prostate hypertrophy.