Tissue inhibitor of metalloproteinases-1 (TIMP-1) is a potent inhibitor of activated matrix metalloproteinases (MMPs) such as gelatinases and collagenases. TIMP-1 is induced by transforming growth factor-1 (TGF-1), but details regarding signaling pathways remain unclear. TH-2 cytokines also have pro-fibrotic properties, and can interact with TGF-. In the present study, we examined the effects of interleukin-13 (IL-13 (2500 pM)) on TGF-1 (200 pM) induced expression of TIMP-1 mRNA and protein in primary human airway fibroblasts from 57 subjects. IL-13 alone had no effect on TIMP-1 mRNA or protein expression. However, IL-13 synergistically augmented TGF-1 induced TIMP-1 mRNA and protein expression (p<0.001, compared to TGF-1 alone). The upregulation of TIMP-1 by the combination of TGF-1 and IL-13 involved increased transcription, with little effect on mRNA stabilization. Initial exploration of the pathways leading to the synergy determined that activation of the phosphatidylinositol 3-kinase (PI3K)/Akt pathway by IL-13 may have a negative effect on TIMP-1 production. The specific PI3K inhibitor, LY294002, in the presence of TGF-1, IL-13 or the combination, caused significant increases in TIMP-1 mRNA expression, while LY294002 increased TIMP-1 protein levels in the presence of IL-13 alone. These results suggest that IL-13 augments TGF-1 induced pro-fibrotic responses at both mRNA and protein level. Although IL-13 induced activation of PI3K/Akt, the activation did not contribute to the synergy observed with TGF-1 plus IL-13 in TIMP-1 expression, and in fact may dampen it. The mechanisms behind the synergy remain to be determined.