Omenn's syndrome is a fatal, autosomal‐recessive combined immune deficiency characterized by several erythematous exfoliative eruptions, lymphadenopathy, hepatosplenomegaly, and elevated eosinophil count. In some of these patients an expansion of CD3⁺CD4⁻CD8⁻ double negative (DN) T cell population was observed. We show here that the DN population represents a clonal expansion of T cells which preferentially use Vβ14 in their T cell receptor complex. Using polymerase chain reaction, we show that patient's DN cells express spontaneously high levels of IL‐5, thus possibly explaining the abundance of eosinophils in this disorder. The increase of IgE observed in patients with Omenn's syndrome is unlikely to be related to IL‐4 production, as IL‐4 levels in patient samples were low. However, patient's low expression of interferon‐gamma (IFN‐γ), which has been reported to inhibit IgE production, may explain the elevated levels of IgE in this patient. The results thus highlight the importance of the inhibitory effect of IFN‐γ on regulation of IgE production.