There have been limited prospective data on prolactin levels and breast cancer risk among premenopausal women [18,19,28] until recently; the Nurses' Health Study reported a non-significant 30% increase in breast cancer risk among premenopausal women when comparing top to bottom quartiles (> 17.6 vs. < 9.8 ng/mL) of PRL levels among 377 cases and 786 controls [29]. Here, PRL is linked to breast cancer.