Medullary breast cancer (MBC) is a basal-like breast carcinoma (BLBC) with a favourable outcome, whereas nonmedullary BLBC has a poor prognosis. Tumour infiltrating lymphocytes (TILs) are present in both MBC and BLBC. We hypothesized that the immunosuppressive enzyme indoleamine 2,3-dioxygenase (IDO) could modulate the TILs effects among these tumours and explain their different outcomes. The amount of TILs and IDO expression were analysed using immunohistochemistry (IHC) in 155 BC cases including MBC (n 5 17), atypical MBC (n 5 13) and non-MBC (n 5 125). Messenger RNA expression of the INDO gene, which encodes IDO, was measured in 262 cases from our institution. INDO mRNA expression and histoclinical data of 1,487 BC cases were collected from public databases. IDO immunostaining was present in both neoplastic and stromal cells in 100% of MBC and was associated with histological medullary features among non-MBC cases. There was a significant correlation between IDO positivity and TIL amounts. In our series including mostly grade-3 BC, IDO immunostaining was the most significant marker (p 5 0.02) associated with better survival in multivariate analysis. Among our 262 analysed BC cases, INDO mRNA showed significant overexpression in BLBC as compared to luminal A tumours, and in MBC as compared to basallike non-MBC. In the pooled series of 1,749 BC cases, INDO mRNA was overexpressed in BLBC and was the most significant predictor of better survival in this subtype using multivariate analysis (p 5 0.0024). In conclusion, high IDO expression is associated with morphological medullary features and has an independent favourable prognostic value in BLBC.Among breast cancer (BC) molecular subtypes, the group of basal-like breast carcinomas (BLBC) is characterised by its particularly poor prognosis. Nonetheless, medullary breast carcinoma (MBC), a histological subtype with favourable prognosis, displays basal-like features.1,2 To clarify this paradox, we had previously performed a gene expression profiling study to discriminate MBC from BLBC and showed that MBC was associated with a specific molecular signature reflecting a TH1 immune response.1 The presence of tumour infiltrating lymphocytes (TILs) is a specific feature of both MBC and BLBC. It is thus possible that these two subtypes delineate a spectrum of BC in which clinical evolution depends on the properties of the lymphoid infiltration.The prognostic significance of lymphoid infiltration in invasive BC remains controversial. [3][4][5][6][7] We have previously observed an association with local recurrence in patients