The effect of flutamide, a potent nonsteroidal antiandrogen, on the metabolism of iv tracers of [3H]estradiol was studied in five patients with advanced prostate cancer. The drug produced no change in the percentage of the injected radioactivity recovered in urine or in the glucuronide or nonglucuronide conjugate fractions. Of the five individual metabolites that were quantitated, estrone, estradiol, and estriol were unaffected by flutamide, but the drug caused striking decreases in conversion of estradiol to 2-hydroxyestrone (4.0% vs. 7.4%) (P less than 0.005) and 2-methoxyestrone (1.1% vs. 2.6%; P less than 0.05); every one of the patients showed a marked fall in recovery of both of these compounds. This depression of the formation of 2-oxygenated metabolites is reminiscent of the findings in liver disease; the same abnormality occurs regularly in cirrhosis and frequently in extrahepatic biliary obstruction. Taken together with our previous studies of the effects of flutamide on testosterone and cortisol metabolism, this study demonstrates that flutamide produces multiple functional, reversible, cirrhosis-like disturbances of steroid metabolism. Because these disturbances are universal in the patients studied regardless of whether they had clinical responses to flutamide, we doubt that the steroid metabolic changes play a role in the therapeutic effect of the drug.