IL-33, a new member of the IL-1 cytokine family, promotes Th2 inflammation, but evidence on the implications of this cytokine in asthma is lacking. IL-33 would be mainly expressed by structural cells, but whether proinflammatory cytokines modulate its expression in airway smooth muscle cells (ASMC) is unknown. Endobronchial biopsies were obtained from adults with mild (n = 8), moderate (n = 8), severe (n = 9), asthma and from control subjects (n = 5). Immunocytochemistry, laser-capture microdissection, reverse transcriptase, and real-time quantitative PCR were used for determining IL-33 expression in the lung tissues. ASMC isolated from resected lung specimens were cultured with proinflammatory cytokines and with dexamethasone. IL-33 expression by ASMC was determined by PCR, ELISA, and Western blotting. Higher levels of IL-33 transcripts are detected in biopsies from asthmatic compared with control subjects, and especially in subjects with severe asthma. ASMC show IL-33 expression at both protein and mRNA levels. IL-33 and TNF-alpha transcript levels correlate in the lung tissues, and TNF-alpha up-regulates IL-33 expression by cultured ASMC in a time- and dose-dependent manner. IFN-gamma also increases IL-33 expression and shows synergistic effect with TNF-alpha. Dexamethasone fails to abolish TNF-alpha-induced IL-33 up-regulation. IL-33 expression increases in bronchial biopsies from subjects with asthma compared with controls, as well as subjects with asthma severity. ASMC are a source of the IL-33 cytokine. Our data propose IL-33 as a novel inflammatory marker of severe and refractory asthma.