Hypomorphic mutations of the RAG genes in humans are associated with a spectrum of clinical and immunologic presentations that range from T- B- severe combined immune deficiency (SCID) to Omenn syndrome. In most cases, residual V(D)J recombination activity allows for development of few T-cell clones, which expand in the periphery and infiltrate target organs, resulting in tissue damage. Invariant natural killer T (iNKT) cells play an important immunoregulatory role and have been associated with protection against autoimmunity. We now report on 5 unrelated cases of combined immune deficiency due to hypomorphic RAG mutations, and demonstrate the absence of iNKT cells in all 5 patients. These findings suggest that lack of this important immunoregulatory cell population may contribute to the pathophysiology of Omenn syndrome.