Five adult asthmatics with nocturnal symptoms (mean FEV1 2.31 1; 2 mean PD20 histamine 1.5 unmoles), 5 asthmatics with no nocturnal symptoms (mean FEV1 l 2.97 l; mean PD20 Histamine 3.7 unmoles) and 5 non-asthmatic control subjects (mean FEV1 3.63 l; mean PD20 histamine 78 unmoles) were challenged with nebulised solutions of their own saliva or isotonic saline in a double blind crossover study to investigate whether the inhalation of saliva during sleep could cause nocturnal asthma. The maximum % fall in FEV1 with saliva was -26.6, -6.5 and -4.1 for the nocturnal, non-nocturnal and control groups respectively. The corresponding values for the maximum % fall in FEV1 with saline was -12.4, -5.5 and -3.6. The difference in maximum % fall in FEV1 with both saliva and saline was significant (p < 0.01) for the nocturnal asthmatics when compared to the non-nocturnal asthmatics. These results lend support to the hypothesis that nocturnal symptoms in asthmatic patients may be triggered by inhalation of saliva during sleep.