We have investigated the role of Th2 cytokines in the development of atopic diseases using transgenic mice carrying large genomic segments containing IL4, IL13 and IL5 genes and overexpressing these Th2 cytokines. In vitro stimulated, but not unstimulated, Th2 cells from the transgenic mice expressed high levels of IL4, IL13 and IL5 compared to those from non-transgenic mice. The transgenic mice developed spontaneous atopic dermatitis and airway inflammation against environmental allergens. The affected regions for atopic dermatitis covered the entire body including skin in the face, ear, eye-lid, neck, hind region and tail. Histological features showed thickened epidermis and dermis and infiltration of large numbers of inflammatory cells in the affected regions. The transgenic mice also showed airway inflammation characteristic of asthma, including infiltration of inflammatory cells and hypertrophy of airway epithelial cells. These mice also expressed high level of serum IgE, which is a hallmark of atopic diseases. In summary, this study provides additional evidence that Th2 cytokines play key roles in atopic diseases.