Previous studies have shown that triggering of Th2 cells via the TCR is sufficient for production of IL-4 but not for proliferation of these cells. Proliferation of Th2 cells occurs only in the additional presence of a costimulatory signal delivered by IL-1. For the majority of Th2 cell clones, this type of proliferation was found to be independent of IL-4. Here, we further investigated the mechanism of IL-4-independent proliferation. We demonstrate that, after costimulation via TCR and IL-1R, but not via either receptor alone, Th2 cells are triggered to produce cell-associated IL-1alpha, as detected at the level of function, protein, and mRNA expression. In the presence of the TCR signal, autocrine IL-1alpha is then able to costimulate IL-4-independent proliferation of Th2 cells and to further enhance its own production. Thus, our results point to a novel, IL-4-independent, self-amplifying autocrine pathway of Th2 cell proliferation that requires a signal via the TCR and a costimulatory signal via IL-1R. This pathway may explain frustrating results in experimental models that attempted to treat established Th2-mediated diseases in vivo with IL-4-neutralizing agents alone.