Recently immunoglobulin G (IgG) was found to be produced by neoplasms and promote tumor growth in cancer cell lines and animal models. To investigate the pathophysiological significance of cancer-produced IgG in breast cancer, we examined the expressions of IgG in 68 breast cancers including 40 primary cancers without metastasis and 28 cancers with axillary lymph node metastases. IgG gene expression was detected in all these samples. We found that IgG-expressing cancer cells were predominantly located in the periphery of the primary cancer nest and that these cells showed more cellular atypia and nuclear pleomorphism. We also found that the abundance of IgG-expressing cancer cells was higher and the cells were more evenly distributed in the metastatic cancer cells than that in the primary lesion. These findings suggest that IgG-expressing breast cancer cells have a more aggressive biological behavior than the IgG negative cancer cells and it could be an indicator for progression and metastasis of the disease. Co-localization of IgG and C1q complement was detected in both primary and metastatic lesions implying that immune complexes might be formed in situ. We speculate that such immune complexes might facilitate immune escape of cancer cells. Our findings suggest that locally produced IgG plays important roles in breast cancer, and may serve as a potential therapeutic target.