The late asthmatic response (LAR) to an allergen challenge has a marked impact on lung function in the patient with asthma. Virtually all studies on the LAR have been done during the daytime. This study evaluated the LAR as a function of the time of day an inhaled allergen challenge was performed. An allergen challenge given in the morning produced a LAR in 4 of 10 subjects, while the same challenge in the evening caused a LAR in 9 of 10 (p less than 0.05). The time to onset of the LAR following the morning and evening challenges was 9.4 +/- 2.0 h versus 3.1 +/- 0.3 h, respectively (p less than 0.05). The maximal decrease in FEV1 for the LAR was 32.8 +/- 5.6% for the morning challenge versus 43.0 +/- 3.1% in the evening (p less than 0.05). Additionally, the bronchial responsiveness to methacholine was significantly greater at 24 h following evening allergen challenge than after the morning (p less than 0.05) challenge. Thus, it is important to take into account the time of day a patient is exposed to an allergen in regard to the development of the LAR.