The basal plasma concentration of prolactin was determined by radioimmunoassay in Caucasian women at different risk for breast cancer and in three cases of women at risk for breast cancer. Samples taken under comparable conditions showed similar basal levels at birth and in prepubertal Bantu and Caucasian girls, or in healthy pre- and postmenopausal Bantu, Caucasian, or Japanese women. Elevated plasma prolactin levels were found in women castrated prior to 35 years of age and in women whose first pregnancy occured after 35 years. Evidence indicates that the prolactin/estrogen relationship may not be similar in women 35 to 45 years of age as compared to young women and suggests that elevation of serum prolactin per se does not appear to be related to an increased risk of breast cancer. The prolactin level was increased only in Caucasian women with breast cancer. If a high prolactin/estradiol ratio increases the susceptibility of the mammary epithelium to neoplastic growth, the lack of changes in prolactin levels in premenopausal Japanese patients and in postmenopausal patients of the three ethnic groups indicates that other factors are involved. Further study of the effects of life-style and diet on the basal level and stimulated release of prolactin is required to resolve the relationship of prolactin to breast cancer.