BACKGROUND: The incidence of female breast cancer has been increasing during last 10 years. There are many risk factors in breast cancer. We evaluated the relative risk of breast cancer in relation to obesity and body composition. Also we evaluated the Bioelectrical Impedance Analysis (BIA) for estimating the obesity. MATERIALS AN METHODS: The data were collected as a case-control study from September 1, 1997 to August 31, 1998. Cases were selected from newly diagnosed and treated breast cancer patients at Breast Clinic of Gil Medical Center, Gachon Medical College. Control group was randomly selected form women with benign breast disease or women in screening rest. All suspected risk factors including obesity indices and body composition were checked. Especially body fat percent, total body water and lean body mass were checked by BIA. RESULTS: There was no association between the relative risk of breast cancer and education, smoking, drinking, oral contraceptives use, reproductive factors and breast feeding factors. Data analysis showed the high possibility of positive relationship between early menarche factors( < or =14 years old) & late menopause ( > or =51 years old ) and the brest cancer, however there was no statistical significance. There was a statistical significance between risk group and control group in fat percent (p=0.0187) and WHR (waist-hip ratio) (p=0.0174) among obesity indicies. BMI & obesity (In the classification if Obesity by Japan Society for Study of Obesity) were associated with relative risk of breast cancer by menopausal status. In premenopausal women, breast cancer risk decreased in high BMI & obese group, however the increase was shown in postmenopausal women contrarily. . CONCLUSIONS: This study suggests that there I a significant difference in the relationship between the obesity index and the relative risk of breast cancer between Korean and Western women. Bioelectrical Impedance Analysis is a available method for estimating of obesity in relative risk of breast cancer women. Further studies will be needed for evaluating the role of BIA and the relationship of obesity with staging & prognosis of breast cancer.