Objective To explore whether alcohol promotes the development of breast cancer in TA2 mice and the possible potential mechanism. Methods Thirty-two 6-8-week old nulliparous female TA2 mice were randomly divided into control and ethanol-exposure groups, 16 mice in each group. The mice of the ethanol-exposure group were given 2% ethanol in drinking water, and the mice of control group received regular drinking water. Serum ethanol concentration in the TA2 mice was measured using an ANALOX AM1 alcohol analyzer. The incidence of breast cancer, tumor growth rate and tumor size of the ethanol-exposure and control groups were observed and compared. The estrogen levels of the two groups was detected by enzyme-linked immunosorbent assay (ELASA).Results Compared with the control group, the tumor formation rate of spontaneous breast cancer in the alcohol-exposure group was significantly increased (62.5% vs. 43.75%, P<0.05), the average number of days of tumor formation was shortened (285 days vs. 335 days, P<0.05), the tumor weight and volume were increased but not significant (P>0.05), and the level of estrogen in the ethanol-exposure mice was significantly higher than that in the control group (P>0.05).Conclusions Alcohol promotes the tumorigenesis of spontaneous breast cancer in TA2 mice, which may be associated to the increase of estrogen levels.