Aspergillus fumigatus is a fungus that is associated with a severe form of asthma, although the precise immunological basis for this disease is unclear. A new study in mice shows that natural killer T (NKT) cells are crucial for progression of A. fumigatus–induced asthma and also identifies a glycolipid antigen from this fungus that seems to drive this NKT cell–mediated inflammatory response (pages 1297–1304).