Abstract: Objective To estimate the association of plasma sex steroid hormone levels with risk of breast cancer among premenopausal women. Methods By use of case-control study，levels of plasma estradiol (E₂)，testosterone (T) and progesterone (P) were measured in 75 premenopausal women with breast cancer patients and 78 matched control subjects by radioimmunoassay during the follicular phase of their menstrual cycles. Conditional logistic regression models, controlling for breast cancer risk factors, were used to examine their associations with breast cancer risk. All statistical tests were two-sided. Results (1)The plasma levels of E₂ and T of case group were significantly higher than those of control group. The plasma level of P was lower than that of the control group,but the resuts as not statistically significant.(2)Increased risks of breast cancer were positively associated with elevated plasma concentration of testosterone(adjusted odds ratioOR for highest versus lowest quartile=3.63, 95% confidence interval95%CI=1.82-7.45, P_trend=0.015) . Elevated serum progesterone concentrations was associated with a statis tically significant reduction in breast cancer risk(adjusted OR=0.43, 95%CI=0.20-0.85, P_trend=0.023). Higher level of estradiol was associated with modest, non-statistically significant increase in risk of breast cancer(adjusted OR=2.48, 95%CI=1.27-5.14,P_trend=0.270). Conclusion Results suggested that high plasma level of testosterone was positively associated with breast cancer risk, while high plasma level of progesterone was negatively associated with breast cancer risk in premenopausal women.