The purpose of this study is to investigate whether the outcome of breast cancer (BC) patients treated with adjuvant chemotherapy is affected by co-mutated TP53 and PIK3CA according to stromal tumor-infiltrating lymphocytes (TILs). Paraffin tumors of all clinical subtypes from 1661 patients with operable breast cancer who were treated within 4 adjuvant trials with anthracycline-taxanes chemotherapy were informative for TP53 and PIK3CA mutation status (semiconductor sequencing genotyping) and for stromal TILs density. Disease-free survival (DFS) was examined. TP53 mutations were associated with higher (p < 0.001) and PIK3CA with lower (p = 0.004) TILs in an ER /PgR-specific manner (p < 0.001). Mutations did not affect the favorable DFS of patients with lymphocyte-predominant (LP) BC. Within non-LPBC, PIK3CA-only mutations conferred best, while TP53-PIK3CA co-mutations (6 % of all tumors) conferred worst DFS (HR 0.59; 95 % CI 0.44-0.79; p = 0.001 for PIK3CA-only). TP53-only mutations were unfavorable in patients with lower TILs, while patients with lower TILs performed worse if their tumors carried TP53-only mutations (interaction p = 0.046). Multivariate analysis revealed favorable PIK3CA-only mutations in non-LPBC (HR 0.64; 95 % CI 0.47-0.88; p = 0.007), and unfavorable TP53 mutations in ER/PgRpos/HER2neg (HR 1.55; 95 % CI 1.07-2.24; p = 0.021). Mutations did not interact with TILs in non-LP triple-negative and HER2-positive patients. TP53 and PIK3CA mutations appear to have diverse effects on the outcome of early BC patients, according to whether these genes are co-mutated or not, and for TP53 according to TILs density and ER/PgR-status. These findings need to be considered when evaluating the effect of these two most frequently mutated genes in the context of large clinical trials.