Expression of IL-5 mRNA and the content of IL-5 in the sputum of patients with asthma of different severity were studied before and after treatment. The expression of IL-5 mRNA in mild asthma differed from that in severe and moderate asthma before and after treatment. The level of IL-5 before therapy was different in patients with mild and severe disease. In patients with severe asthma the level of IL-5 differed before and after treatment.