We evaluated a clinical significance of indoleamine 2,3-dioxygenase (IDO) in thyroid cancers. Operative specimens obtained from 20 patients with thyroid cancers were investigated by semi-quantitative RT-PCR with specific primers against IDO. The correlations among IDO expression, clinicopathologic factors and prognosis were studied. An expression of IDO was observed in 100% for both of the cancer specimens and the non-cancer specimens. The IDO expression of cancer specimens was significantly 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 it was correlated to clinical stage. There was no correlation for survival rate after surgery between the high IDO level group and low IDO level group. The serum IDO levels of cancer patients were higher than that of a healthy volunteer measured by semi-quantitative RT- PCR and HPLC. It is suggested that the expression of IDO in thyroid cancer patients may play critical role for immunosuppression of those patients.