The effect of a 1-hour exposure at rest during passive cigarette smoking (20 ppm CO) or Sham was investigated in 11 children with bronchial asthma (age range, 8-13 yr; ten boys, one girl). Nine of the subjects were on regular therapy with inhaled beta 2-agonists and disodium cromoglycate. Both drugs were withheld at least 6 hours prior to each study session. Exposure was performed in an environmental chamber. Before and immediately after exposure, lung function and symptom scores were determined. After exposure, a histamine inhalation challenge was performed to determine the concentrations that caused a 100% increase in SRaw (PC100SRaw) and a 20% fall in FEV1, (PC20FEV1). 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) L, respectively. Before and after cigarette 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 L, respectively. Geometric mean (SD) PC100 SRaw and PC20FEV1 after Sham was 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 statistical differences in lung function and PC values between Sham and passive cigarette smoking. The main symptoms during passive smoking were irritation of the eye and the nasopharynx. Our observations suggest that in children with mild bronchial asthma 1 hour of passive cigarette smoking does not cause consistent changes of lung function and bronchial responsiveness.