A study of 424 women was undertaken to determine whether there was an association between serum prolactin levels and breast cancer; whether prolactin levels would reflect degrees of risk of developing breast cancer; and whether associations between known risk factors for breast cancer and serum prolactin concentrations could be demonstrated. Prolactin levels higher than the median value in control subjects were found to be associated with a more than two-fold increase in the risk of breast cancer (relative risk, 2.1; confidence interval [CI], 1.0-4.5). Moreover, a relative risk of 1.7 (CI, 0.9-3.3) for a group of women with benign epithelial hyperplasia (high risk of developing breast cancer), and a relative risk of 1.0 (CI, 0.6-1.8) for a group with benign fibrocystic disease (low risk of developing breast cancer), provided supportive evidence that prolactin plays a role in the development of breast cancer. A considerable fall in the concentration of prolactin at menopause was noted, so those women who have an early menopause have a reduced period of exposure to high concentrations of prolactin. Similarly, there was a considerable reduction in prolactin concentration after the first pregnancy. Finally, our results showed that, in premenopausal women, a high intake of saturated fats was associated with a high prolactin concentration. Our study supports the concept that parity, menstrual status, and saturated fat consumption influence a woman's exposure to prolactin and therefore the risk of developing breast cancer.