NKT cells are significantly reduced in the blood of newly diagnosed breast cancer (BC) patients compared to healthy volunteers (HV). We hypothesize that one mechanism by which breast cancers escape immune detection is due to a loss in NKT cell number and function. Therefore, we used mice genetically predisposed to develop breast cancer (BRCA1-mut) to examine the phenotype and function of NKT cells throughout tumorigenesis. We found that BRCA1-mut mice have a 70-85% reduction of NKT cells in primary and secondary lymphoid organs, and that this reduction occurs in stage 3 of development. Moreover, the NKT cells of BRCA1-mut mice are functionally altered. Thus, our data suggest that NKT cells play a critical role in the development of breast tumorigenesis and further research is required to determine the mechanisms by which NKT cells are physically and functionally reduced in the absence of BRCA1.