Lymph node status is a key indicator of the best approach to treatment of invasive breast cancer. However, the accuracy with which lymph node metastasis is diagnosed is not currently satisfactory. New and more reliable methods that enable one to know who has a greater potential for lymph node metastasis would be highly desirable. We previously reported that lymph node involvement in esophageal and lung cancer may have a genetic component: C-reactive protein (CRP) 1846C>T genetic polymorphism. Here we examined the diagnostic value of CRP 1846C>T polymorphism for assessing the risk of lymph node metastasis in cases of invasive breast cancer. The study participants were 185 women with invasive breast cancer who underwent curative surgery with lymph node dissection. Using DNA from blood samples and polymerase chain reaction-restriction fragment length polymorphism, the utility of CRP genetic 1846C>T polymorphism (rs1205) for assessing the risk of lymph node metastasis was evaluated. Fifty-two (28 %) patients had lymph node metastasis. After the patients were divided into two groups based on their CRP 1846 genotypes (C/C+C/T and T/T), the clinical characteristics did not differ between the groups, but there was a significantly greater incidence of lymph node metastasis among patients in the T/T group. Moreover, the odds ratio for lymph node involvement in patients carrying the 1846 T/T genotype was more than 2.2 in multivariate logistic regression models. CRP genetic polymorphism may be a novel predictor of the risk of lymph node metastasis in invasive breast cancer.