Blood and urine specimens from 27 premenopausal breast cancer patients and 62 healthy controls have been compared with respect to concentration of testosterone and progesterone in blood and of testosterone and androstanediol in urine, measured in the luteal phase of the menstrual cycle. There was a strong positive association between the concentration of the two androgens, either in blood or urine, and breast cancer risk. A strong association was also observed with decreasing levels of progesterone. The association was statistically significant (p for trend less than 0.01) for each hormone; the rate ratios were 10.2 for serum testosterone (highest category), 5.6 for serum progesterone (lowest category), 8.4 for urinary testosterone (highest category), and 5.2 for androstanediol (highest category). The rate ratio for women presenting both high serum testosterone and low progesterone was 21.8 (4.1 to 116.1). Considering the exposure to at least one of three androgens at the highest level and low progesterone, the rate ratio was as high as 90.2 (8.2 to 989.7). This study provides evidence for the hypothesis that increased androgenic activity is an important risk indicator for breast cancer, particularly when associated with anovulation, as indicated by low serum progesterone level.