The biotransformation of radioactive estradioactive estradiol and estrone was examined in 10 studies of 6 men with breast cancer. Five normal healthy men of comparable age, 9 men with various cancers unrelated to either breast or endocrine system, 1 patient with heart disease and 1 patient with gynecomastia were similarly examined for the pattern of estradiol metabolism. Compared with the well or ill control subjects, the men with breast cancer exhibited sharp diminution in the formation of estrone, 2-hydroxyestrone and 2-methoxy-estrone, accompanied by pronounced increase in the production of estriol. The results reported imply a relationship between estrogen biotransformation and an unusual, steroid hormone-related malignancy.