AIMS: The mechanisms underlying cardiac fibrosis in hypertension are yet to be defined, although inflammatory cells, fibroblasts, and cytokines have been implicated. Here, we investigated the role of interleukin-4 (IL-4) in cardiac fibrosis, which is elevated in the hypertensive heart. IL-4 has been shown to be pro-fibrotic in the liver and the lung, but its role in cardiac fibrosis has not been investigated. METHODS AND RESULTS: Cardiac fibrosis was induced in mice by constricting the aorta between the two carotid arteries. Fourteen days later marked left ventricular fibrosis developed together with expression of IL-4. Anti-IL-4 neutralizing antibodies attenuated this fibrosis without affecting blood pressure or expression of the transforming growth factor-beta system. The reduction in fibrosis was associated with reductions in interstitial fibroblasts and macrophages together with reductions in proliferating cells and expression of monocyte chemoattractant protein-1 (MCP-1). Since mast cells are a source of IL-4, we also assessed their role in fibrosis. Cromolyn, a mast cell inhibitor attenuated mast cell degranulation as well as IL-4 mRNA expression and cardiac fibrosis without affecting blood pressure. Treatment with Cromolyn also reduced interstitial fibroblasts and macrophages in regions of developing fibrosis as well MCP-1 expression. CONCLUSION: This study demonstrates for the first time that IL-4, most likely produced by mast cells in the heart during pressure overload, is a significant contributor to cardiac fibrosis. Targeting this cytokine may be a useful therapeutic strategy to limit cardiac fibrosis.