Objective To investigate the effect of ethanol on radiosensitivity of human breast cancer MCF-7 cells.Methods Human breast cancer MCF-7 cells were divided into four groups including control group,ethanol treatment group,X-ray exposed group,and ethanol combined with X-ray group.Clonogenic assay was used to determine cell survival.Flow cytometry was employed to analyze cell cycle progression.Annexin V-FITC kit was used to determine cell apoptosis induction.Results Ethanol(50 and 100 mmol/L,50 h)had no influence on MCF-7 cell growth( t =0.82,1.15,P ＞0.05 ).The radiosensitivity of MCF-7 cells was reduced when the cells were pretreated with 50 mmol/L ethanol (t =4.15,P ＜0.05)and 100 mmol/L ethanol ( t =10.28,P ＜ 0.05 ) for 2 h. Compared with irradiation with X-ray alone,ethanol treatment decreased G2/M phase arrest(t =7.18,P ＜0.05) and sub-G1population(an indicator of apoptosis induction) ( t =5.39,P ＜ 0.05).A decrease of advanced and early apoptosis in the cells pretreated with ethanol was also confirmed by Annexin V-FITC apoptosis assay( t =4.86,7.59,P ＜ 0.05 ).Conclusions Ethanol causes radioresistance in human breast cancer MCF-7 cells,where the decreases of radiation-induced G2/M phase arrest and apoptosis may be involved.