Using our syngeneic mouse model of breast cancer metastasis, we show that tumor cells expressing CD1d promote increased iNKT-mediated antitumor immunity in a CD1d-dependent manner in vitro and in vivo. Importantly, inhibition of tumor CD1d expression in vivo, by either antibody blockade or gene silencing, promotes spontaneous breast cancer metastasis. Here, CD1D is linked to breast cancer.