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. The gene discussed is CD1D; the disease is breast carcinoma.