To study the effect of environmental tobacco smoke (ETS) exposure in the evening on nocturnal changes in airway tone and responsiveness, 17 subjects with mild asthma (mean +/- SD age, 26 +/- 5 years, FEV1% pred., 89 +/- 14%) were exposed to either ambient air (sham) or ETS (20 ppm CO) for 3 h (7:00 to 10:00 p.m.). Seven subjects had a history of ETS-induced respiratory symptoms. Spirometry was performed 2 h before exposure (5:00 p.m.), every 30 min during exposure, and at 11:00 p.m., 3:00 a.m., and 7:00 a.m. The provocative concentrations of methacholine necessary to decrease FEV1 by 20%, PC20FEV1, were assessed at 5:00 p.m., 11:00 p.m., 3:00 a.m., and 7:00 a.m. Compared with pre-exposure measurements, mean FEV1 values during and after ETS exposure were significantly lower than with sham exposure = 0.013 and 0.026). This effect, however, was due to a significant response in single individuals. The higher bronchial responsiveness after ETS than after sham exceeded one doubling concentration in 4, 5, and 4 patients at 11:00 p.m., 3:00 a.m., and 7:00 a.m., respectively. The opposite effect was observed in 2, 2, and 2 patients, respectively. There was no statistically significant mean effect of ETS on airway responsiveness during night; however, there was significant heterogeneity in individual responses (P = 0.0002). Patients with and without a history of ETS-induced symptoms did not show different responses to experimental ETS exposure. In conclusion, our data suggest that in single adult subjects with mild asthma, acute exposure to ETS in the evening can produce a deterioration of airway tone and responsiveness during the night, with wide interindividual variability in the response.