Nocturnal asthma is a frequent problem, but the mechanism is unclear. We investigated the possibility that airways inflammation occurred during the night. Bronchoalveolar lavage fluid was analyzed in asthmatic patients with (n = 7) and without nocturnal asthma (n = 7) at 1600 and 0400 h. The nocturnal asthma group had an increase in the total leukocyte count (24.0 ± 7.0 to 41.1 ± 9.9 × 10⁴ cells/ml, p < 0.05), neutrophils (1.1 ± 0.6 to 3.7 ± 1.5 #x00D7; 10⁴ cells/ml, p < 0.05), and eosinophils(0.5 ± 0.1 to 1.7 ± 0.7 × 10⁴ cells/ml, p < 0.05) from 1600 to 0400 h. Cellular components for the non-nocturnal asthma group did not change. Between groups, the 1600-h cells were similar. At 0400 h the nocturnal asthma group had significantly higher total leukocyte, neutrophil, eosinophil, lymphocyte, and epithelial cell counts. For all subjects, the overnight fall in peak expiratory flow rates was correlated to the change in neutrophils (r = 0.54, p < 0.05) and eosinophils (r = 0.77, p < 0.05). We conclude that the nocturnal worsening of asthma has an associated cellular inflammatory response that is not seen in patients without overnight decrements in lung function. This inflammatory response together with epithelial damage may be important factors in the etiology of nocturnal asthma.