We evaluated the clinical significance of indoleamine 2,3-dioxygenase(IDO)in intra-cystic tumors of the breast. IDO activitycan be measured byusing the tryptophan(Trp)/kynurenine(Kyn)ratio. Trp and Kyn were measured by high performance liquid chromatography(HPLC). We took liquid from 3 intra-cystic papilloma cases and 5 intra-cystic breast cancer cases. Then, we measured the Trp/Kyn ratio in these samples, and we evaluated the difference in the Trp/Kyn ratio between the papilloma group and the breast cancer group. The Trp/Kyn ratio was significantly lower in the breast cancer group than in the papilloma group. These results suggest that measuring the Trp/Kyn ratio in cystic liquid from intra-cystic breast lesions is useful to distinguish benign intra-cystic lesions from malignant intra-cystic lesions.