BACKGROUND: There have been very few studies on the relationship between obesity and dense breast as risk factors of breast cancer in Korean women. Therefore, we evaluated the relationship between BMI and mammographic density in Korean women. METHODS: We recruited 332 women without prior history of breast cancer, breast surgery, radiotherapy, hormone replacement therapy, and family history of breast cancer who underwent screening for mammography and completed a self-administered questionnaire in one general hospital. On the basis of ACR BI-RADS breast composition, four density patterns were classified repeatedly by a radiologist. RESULTS: The mean age, Body Mass Index (BMI), the proportion of postmenopausal women of the study population were 50.4 years, 23.2 kg/m2, and 29.8%, respectively. The proportion of extremely dense breast women was 100% in the underweight group, 77.3% in the normal weight group, 41.4% in class 1 obesity group, and 25% in class 2 obesity group, decreased with increasing BMI. The odds Ratio of BMI to extremely dense breast was 0.82 (95% CI: 0.72~0.93, P value=0.002) after adjustment for age, menache, menopause, and BI-RADS assessment category in logistic regression. CONCLUSION: Our findings showed that the BMI was negatively correlated with mammographic density as well in Korea. Therefore, primary physicians should consider sensitive additional method for breast cancer screening especially in underweight and normal weight Korean women.