Cytokine production by T cells and eosinophils plays a major role in allergic asthma. Both cell types produce interleukin (IL)-9, which belongs to the group of T-helper type 2 cytokines comprising IL-4, IL-5, and IL-13. It shares genetic as well as functional similarities with these cytokines and may therefore contribute to the pathogenesis of allergic disorders such as bronchial asthma. To investigate the relevance of IL-9 for the formation of the allergic response, we measured the expression of IL-9 messenger RNA of cells obtained from BAL before and 24 h after segmental allergen and saline solution challenge of 10 subjects with mild asthma and 5 healthy control subjects. Following processing of cells, messenger RNA expression of IL-9 and porphobilinogen deaminase as a housekeeping gene was measured using real-time polymerase chain reaction with the Lightcycler (Roche Diagnostics; Mannheim, Germany). Relative quantification of IL-9 messenger RNA was performed vs the housekeeping gene porphobilinogen deaminase.