Abstract: Objective:To estimate the association of plasma prolactin(PRL) concentration with the risks of breast cancer among premenopausal women.Methods:The concentration of plasma PRL was measured in 65 premenopausal women with breast cancer and 65 matched control subjects by radioimmunoassay.The relations between the plasma PRL concentration and the breast cancer in premenopausal women were observed using conditional logistic regression analysis.All the cases were grouped according to the pathological features;the association of the level of PRL and the risk of breast cancer for different sub-groups were evaluated.Results:The plasma PRL concentration of the case group was significantly higher than that of the control group.Increased risk of breast cancer was positively related with the elevated plasma concentration of PRL(adjusted odds ratio OR for the highest versus the lowest quartile=1.54, 95% confidence interval 95%CI=0.84-4.07, Ptrend=0.031).The concentration of plasma PRL increased modestly in estrogen receptor positive carcinomas but did not change with the tumor size, grade or lymph node status.Conclusions:The results suggest that high plasma concentration of PRL is positively related with the breast cancer, especially estrogen receptor positive carcinomas among premenopausal women.