IL-12 and IL-13 are recently discovered cytokines which belong to Th1 and Th2 subtypes, respectively. To further support the hypothesis that Th2-type cytokines are involved in asthma, mRNA levels for IL-12 and IL-13 were measured in bronchial biopsies from asthmatics and normal controls as well as in moderately severe asthmatics before and after corticosteroid therapy using the technique of in situ hybridization. The number of IL-13 mRNA positive cells in asthmatics was significantly higher than that found in normal controls. However, there was a significant decrease in the number of IL-12 mRNA positive cells in asthmatic biopsies compared to normal controls. Following corticosteroid therapy, a significant decrease in IL-13 mRNA positive cells and an increase in the number of cells expressing IL-12 mRNA was detected. The results suggest an in vivo role of the cytokines in modulating allergic responses and support the role of Th2 type cytokines in asthma.