Background: Breast carcinomas (BC) are often considered to be weakly immunogenic and thus poorly sensitive to immunotherapy. Methods: We analyzed the repertoire of tumor-infiltrating T cells (TILs) in 41 early BC by sequencing their T cell receptor β genes (TCRβ). Libraries were built using a digital sequencing approach, barcoding each sequenced molecule to improve accuracy and quantification. T cell repertoires were also obtained from paired blood samples allowing identification of T cell clones enriched in the tumors as compared to blood. For 5 patients, CD8⁺ TILs were cloned ex-vivo from a tumor sample and screened for recognition of autologous predicted neoepitopes. Results: T cell infiltration differed from one tumor to another. Its amount varied of more than 30 fold and its diversity ranged from < 100 to > 5000 different clonotypes. In 34% of the tumors, there was an important T cell infiltration and we detected several clonotypes with a ≥500 fold enrichment as compared to blood. In 22% of the tumors, an important T cell infiltration was observed but without significantly enriched clonotypes. In 43% of the tumors the T cell infiltration was very limited. For 5 tumors with a high T cell infiltration, we screened ex-vivo isolated CD8⁺ T cell clones for recognition of predicted neoepitopes. In 4 of these tumors, with no enriched clonotypes, no recognition was observed. In 1 of these tumors, with enriched clonotypes, 6 different CD8⁺ T cell clones recognized 4 predicted neoepitopes. Three of these clones were > 100 fold more frequent in the tumor as compared to blood. Conclusions: About 30% of early BC were infiltrated by T cell clonotypes significantly enriched relative to blood. In one of these tumors some of the most enriched clonotypes recognized neoepitopes, demonstrating that some primary BC are spontaneously immunogenic. About 20% of the tumors had an important T cell infiltration without enriched clonotypes. None of the TIL clones isolated from 4 such tumors recognized predicted neoepitopes. Our results suggest that the detection of intratumorally enriched T cell clonotypes could identify immunogenic tumors, which may be sensitive to treatment with immunostimulatory antibodies.