cAMP is an important second messenger with immunomodulatory properties. Elevation of intracellular cAMP in T cells, induced by agents such as IL-1alpha or PGs, inhibits T cell activation. In effector T cells, an increase in the level of intracellular cAMP inhibits cytokine production in Th1 cells but stimulates cytokine production in Th2 cells. Here we report that cAMP-induced effects in Th2 cells occur independently of the protein kinase A pathway, which is the major mediator of cAMP-induced signaling events in most cell types. Instead, cAMP stimulates activation of p38 mitogen-activated protein kinase in Th2 cells. This appears to be a Th2-selective event because cAMP barely increased p38 phosphorylation in Th1 cells. We show that in Th2 cells, cAMP promotes the production of both IL-5 and IL-13, which play distinct but critical roles in asthma pathogenesis. Our data also show that cAMP causes increased phosphorylation of the transcription factor GATA-3, which we have shown is a critical regulator of Th2 cytokine gene expression and, in turn, of airway inflammation in mice. Thus, Th2-specific GATA-3 expression and p38 mitogen-activated protein kinase activation together provide a molecular basis for the differential effects of cAMP in the two T helper cell subsets.