We evaluated the clinical significance of indoleamine 2,3-dioxygenase (IDO) in breast cancer. Operative specimens obtained from 30 patients with breast cancer were investigated by semiquantitative RT-PCR with specific primers against IDO. The correlations among IDO expression, clinicopathologic factors and prognosis were studied. The expression of IDO was observed in 100%, both of the cancer specimens and the non-cancer specimens. The IDO expression of the cancer specimens was higher than the non-cancer specimens. The expression of IDO did not correlate to histologic classification, tumor size, lymphatic invasion, venous invasion and lymph nodes metastasis, but correlated to clinical stage and the serum level of immunosuppressive acidic protein (IAP). There were no correlations for a survival rate after surgery between the high IDO level group and the one. The serum IDO levels of cancer patients were higher than that of a healthy volunteer measured by semiquantitative RT-PCR and HPLC. It is suggested that the expression of IDO in breast cancer patients may play a critical role for immunosuppression in those patients.