Obesity is one of the well-known risk factors of breast cancer. We evaluated the relationship between serum adiponectin and resistin levels and breast cancer risk in 41 biopsy-proven breast cancer patients and 43 age- and body mass index-matched controls. The mean serum adiponectin level was lower in the breast cancer group than the control group (6.93+/-3.2 microg/mL, vs. 7.60+/-3.5 microg/mL), but this difference did not reach statistical significance (p=0.37). There was a statistically significant difference in serum resistin levels between the groups (breast cancer group 5.23+/-6.9 ng/mL vs. control 1.46+/-2.0 ng/mL; p<0.001). The risk of breast cancer was significantly increased in the highest tertile group for serum resistin level compared to the lowest tertile group (adjusted odds ratio 2.77 [95% CI 1.40-5.50]). The lymph node metastasis was significantly increased in the patients with less than the median adiponectin level (p=0.017). In the patients whose resistin level was higher than the median, the frequency of tumor with the highest histological grade was significantly increased (p=0.025). In conclusions, both the low serum adiponectin levels and high resistin levels are likely to be associated with increased breast cancer risk in Korean women.