Interleukin-4 (IL-4) is an important cytokine in the allergic inflammation associated with atopic asthma. Interleukin-13 shares many of the biological effects of IL-4, and the evidence suggests that the expression of these two cytokine genes may be coregulated. We have investigated the expression of IL-13 and IL-4 mRNA in the bronchial mucosa of nine stable atopic asthmatics and 10 normal controls, characterized the major cellular source of IL-13 mRNA, and examined the colocalization of IL-13 and IL-4 mRNA. Endobronchial biopsies were obtained and examined for IL-13 and IL-4 mRNA using radiolabeled in situ hybridization. The number of positive cells per millimeter of basement membrane for both IL-13 and IL-4 mRNA was increased significantly in the bronchial mucosa of atopic asthmatics compared to normal controls (p<. 001). In the atopic asthmatics, the expression of IL-13 was significantly greater than that for IL-4 (p<. 01). In these subjects, 90% of the IL-13 mRNA-positive cells were CD3-positive T cells. Furthermore, although 100% of IL-4-positive cells also expressed IL-13 mRNA, only 60% of IL-13-positive cells also expressed IL-4. These results demonstrate that, in mild atopic asthma, IL-13 and IL-4 are coexpressed and that the upregulation of IL-13 expression is greater than that of IL-4. Our data support the role of IL-13 in the allergic inflammation present in atopic asthma.