Human invariant V alpha 24 natural killer T (NKT) cells are a novel, distinct lymphocyte population, characterized by an invariant T-cell receptor V alpha 24 chain paired with V beta 11. V alpha 24 NKT cells are activated by a specific glicolipid ligand, alpha-GalCer, and rapidly produce a large amount of Th1 and Th2 cytokines, thereby modulating other immune cells such as antigen-specific CD4 and CD8 T cells, NK cells, and dendritic cells. Recent studies have shown that NKT cells play pivotal regulatory roles in many immune responses, including antitumor immunity. We herein review the quantitative alteration and functional deterioration of circulating V alpha 24 NKT cells in various cancer-bearing patients. We also summarize the recent progress in the clinical studies of NKT cell-based tumor immunotherapy. Novel immunological results including the increased peripheral blood V alpha 24 NKT cells and IFN-producing cells after the immunotherapy were revealed. The details of the safety profile and the antitumor responses were also disclosed. Although the objective clinical responses still remain unclear, some encouraging results have emerged. Therefore, NKT cell-based immunotherapy may potentially be an effective strategy for the treatment of cancer patients.