30 postmenopausal patients with metastatic breast cancer were treated with three different doses of fadrozole hydrochloride (CGS 169 49A), a non-steroidal competitive aromatase inhibitor. The effect of 0.5, 1 and 2 mg given twice daily upon the levels of oestrogens, their androgen precursors and upon the concentration of sex hormone binding globulin (SHBG) was investigated after 1 and 3 months and then every 3 months until progression of disease. A significant reduction in the serum concentration of oestrone (P < 0.0001) was obtained at all doses. Also, the serum concentration of oestrone sulphate was significantly reduced (P < 0.001). However, after 1 month, the concentration was significantly different from pretreatment levels (P < 0.01) only at the 4 mg daily dose. A decline was also observed in the concentration of SHBG (P < 0.05), with a concomitant elevation of the percentage non-SHBG-bound oestradiol. The androgens, testosterone and dehydroepiandrosterone sulphate, were unaltered during treatment, while androstendione was significantly elevated at the 2 mg daily dose (P < 0.001).