IL‐4 plays a crucial role in the pathogenesis of allergic diseases, such as the induction of IgE synthesis and the development of mast cells. To further understand the effect of IL‐4 on mast cells in skin, we utilized a mast cell/fibroblast co‐culture system as an in vitro model of dermal mast cells. IL‐4 induced mast cell growth in the culture with fibroblasts. Immunoblot analysis revealed that IL‐4 activated Stat6 in both mast cells and fibroblasts. The over‐expression of dominant‐negative Stat6 in fibroblasts in the presence of IL‐4 decreased the histamine content per mast cell, but not the number of mast cells. In contrast, the over‐expression of constitutively‐active Stat6 in fibroblasts increased the histamine content per mast cell, indicating that the activation of Stat6 in fibroblasts supports the maturation of mast cells co‐cultured with fibroblasts.