We exposed 18 adults with bronchial asthma, 16 healthy controls and 11 children with asthma for 1 h either to ambient air (AA) or to environmental tobacco smoke (ETS). Exposure was performed at rest in an exposure chamber. Before and after exposure symptom scores and lung function were determined. After exposure bronchoprovocation tests with methacholine (adults) or histamine (children) were performed to determine the concentrations causing a 100% increase in SRaw (PC100SRaw), and a 20% fall in FEV1 (PC20FEV1). In adult asthmatics mean (SD) SRaw before and after Sham was 8.8 (3.6) and 8.4 (3.6) cmH2O.s, and mean FEV1 (SD) was 3.18 (0.97) and 3.14 (0.9) 1, respectively. Before and after passive smoking mean SRaw (SD) was 7.5 (3.0) and 7.2 (2.7) cmH2O.s, and mean FEV1 (SD) was 3.31 (1.0) and 3.21 (0.88) 1, respectively. Geometric mean (SD) PC100SRaw and PC20FEV1 after Sham were 0.38 (4.5) and 0.29 (4.1) mg/ml, after passive smoking 0.39 (5.1) and 0.36 (4.7) mg/ml, respectively. In healthy controls there was no consistent effect on the respective parameters during exposure. In children mean (SD) SRaw before and after Sham was 8.7 (3.6) and 9.0 (3.2) cmH2O.s, and mean FEV1 (SD) was 1.97 (0.32) and 1.98 (0.40) 1, respectively. Before and after passive smoking mean SRaw (SD) was 10.4 (5.3) and 9.4 (3.3) cmH2O.s, and mean FEV1 (SD) was 1.95 (0.37) and 1.94 (0.35) 1, respectively. Geometric mean (SD) PC100SRaw and PC20FEV1 after Sham were 1.39 (3.0) and 0.70 (2.7) mg/ml, and after passive smoking 1.65 (2.5) and 0.96 (2.3) mg/ml, respectively. There were no significant differences in lung function and airway responsiveness between exposure to ambient air or ETS.(ABSTRACT TRUNCATED AT 250 WORDS)