Background:Previous prospective studies have found an association between prolactin (PRL) levels and increased risk of breast cancer. Using data from a population-based breast cancer case-control study conducted in two cities in Poland (2000-2003), we examined the association of PRL levels with breast cancer risk factors among controls and with tumour characteristics among the cases.Methods:We analysed PRL serum levels among 773 controls without breast cancer matched on age and residence to 776 invasive breast cancer cases with available pretreatment serum. Tumours were centrally reviewed and prepared as tissue microarrays for immunohistochemical analysis. Breast cancer risk factors, assessed by interview, were related to serum PRL levels among controls using analysis of variance. Mean serum PRL levels by tumour characteristics are reported. These associations also were evaluated using polytomous logistic regression.Results:Prolactin levels were associated with nulliparity in premenopausal (P = 0.05) but not in postmenopausal women. Associations in postmenopausal women included an inverse association with increasing body mass index (P = 0.0008) and direct association with use of recent/current hormone therapy (P = 0.0006). In case-only analyses, higher PRL levels were more strongly associated with lobular compared with ductal carcinoma among postmenopausal women (P = 0.02). Levels were not different by tumour size, grade, node involvement or oestrogen receptor, progesterone receptor, or human epidermal growth factor receptor 2 status.Conclusions:Our analysis demonstrates that PRL levels are higher among premenopausal nulliparous as compared with parous women. Among postmenopausal women, levels were higher among hormone users and lower among obese women. These results may have value in understanding the mechanisms underlying several breast cancer risk factor associations. © 2010 Cancer Research UK. All rights reserved.
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