Résumé : Background: Tumour infiltrating lymphocytes (TILs) is one of the most exciting breast cancer biomarkers, yet no data is available on its prevalence in tumours diagnosed during pregnancy. Methods: We evaluated the prevalence of TILs (stromal and intratumoural) in pregnant and non-pregnant young breast cancer patients. Results: 11/116 (9.6%) of the non-pregnant and 2/86 (2.3%) pregnant patients had TILs≥50% (p<0.001) with highest prevalence observed in triple negative tumours (p=0.01). Conclusions: This is the first report on TILs in tumours diagnosed during pregnancy. The low prevalence could reflect the state of low host immunity associated with pregnancy.