Patients with Wiskott-Aldrich syndrome (WAS) have numerous immune cell deficiencies, but it remains unclear how abnormalities in individual cell types contribute to the pathologies of WAS. In T cells, the WAS protein (WASp) regulates actin polymerization and transcription, and plays a role in the dynamics of the immunologic synapse. To examine how these events influence CD4 function, we isolated the WASp deficiency to CD4⁺ T cells by adoptive transfer into wild-type mice to study T-cell priming and effector function. WAS^−/− CD4⁺ T cells mediated protective T-helper 1 (Th1) responses to Leishmania major in vivo, but were unable to support Th2 immunity to Nippostrongylus brasiliensis or L major. Mechanistically, WASp was not required for Th2 programming but was required for Th2 effector function. WAS^−/− CD4⁺ T cells up-regulated IL-4 and GATA3 mRNA and secreted IL-4 protein during Th2 differentiation. In contrast, cytokine transcription was uncoupled from protein production in WAS^−/− Th2-primed effectors. WAS^−/− Th2s failed to produce IL-4 protein on restimulation despite elevated IL-4/GATA3 mRNA. Moreover, dominant-negative WASp expression in WT effector T cells blocked IL-4 production, but had no effect on IFNγ. Thus WASp plays a selective, posttranscriptional role in Th2 effector function.