Natural killer T (NKT) cells play a critical role in anti-tumor immunity. While clinical trials focused on NKT cell modulation have had limited efficacy, the best patient responders to NKT cell-based immunotherapeutic strategies are those with detectable levels of functional NKT cells. Therefore, a specific and highly sensitive assessment of NKT cell function should be performed prior to the initiation of immunotherapy.  We have developed a novel cell-based molecular diagnostic for quantitation of a patient’s NKT cell activity by monitoring IFN-γ induction.  We have measured NKT cell number and function in healthy donors, breast cancer patients, and lymphoma patients. We found that in certain types of cancer, NKT cell number does not correlate with function.  In addition, we have identified immune gene signatures and serum biomarkers that correlate with a reduction in circulating NKT cell function.   Collectively, these data highlight the need for assessing immune cell function preceding treatment, and further suggest that NKT cell-based immunotherapeutic strategies may be more effective in specific types of cancer.