Higher concentrations of prolactin were significantly associated with in situ breast cancer diagnosed less than 4 years since blood donation (ORlog2 = 1.78 (95% CI 1.12, 2.84), Ptrend = 0.01), but not with breast cancer diagnosed 4 or more years since blood donation (ORlog2 = 1.09 (95% CI 0.77, 1.55), Ptrend = 0.63; Phet = 0.04). Here, PRL is linked to breast cancer.