We studied the acute effect of passive smoking on symptoms, lung mechanics and airway responsiveness. Twenty four patients with mild to moderate bronchial asthma (11 male and 13 female; mean(SD) age 34(15) yrs; forced expiratory volume in one second (FEV1) 91(17) % pred) were investigated. Sixteen of them had a history of passive smoke-induced respiratory symptoms. For comparison we studied 16 controls (7 male and 9 female; mean(SD) age 31(9) yrs; FEV1 106(13) % pred). On two different days, the subjects were exposed in an exposure chamber for one hour to either ambient air (Sham) or environmental tobacco smoke (ETS). During exposure to ETS, the mean concentrations of particles and CO were 3,095 micrograms.m-3 and 20.3 ppm, respectively. Before and immediately after exposure, symptoms and lung mechanics were assessed, followed by an inhalation challenge to determine the provocative concentrations of methacholine necessary to increase specific airway resistance (sRaw) by 100%, (PC100sRaw), and to decrease FEV1 by 20% (PC20FEV1). In the asthmatic subjects, during Sham exposure, mean (SEM) decrease of sRaw and FEV1 was 0.23(0.22) cmH2O.s and 0.04(0.03) l, respectively, (NS). During ETS, mean(SEM) decrease of sRaw and FEV1 was 0.55(0.46) cmH2O.s and 0.13(0.06) l, respectively. The significance of this decrease, however, disappeared when taking into account the individual variability of FEV1. Geometric mean(SEM) PC100sRaw and PC20FEV1 were 0.35(1.32) and 0.23(1.34) mg.ml-1 after Sham, and 0.34(1.37) and 0.28(1.36) mg.ml-1 after ETS, respectively, with no difference between the two study days. In the controls, the two exposure conditions did not exert any significant effects on sRaw, FEV1 and airway responsiveness.(ABSTRACT TRUNCATED AT 250 WORDS)