Pulmonary function and symptom responses of asymptomatic smokers were measured during chamber exposures to ozone or filtered air. Subjects were evaluated while habituated to smoking and after 6 mo of smoking abstinence. At the onset of treatment, subjects (n = 18) who had a history of smoking 33.4 +/- 15.6 packs/y and who had almost normal pulmonary function (forced vital capacity [FVC], forced expiratory volume in 1 s [FEV1.0], and mid-maximal flow [MMF] were greater than 82% of predicted) were unresponsive to ozone exposure (0.4 ppm for 2 h). A matched group of control smokers (n = 16) did not respond to filtered air exposure. After 6 mo of smoking cessation, baseline MMF rates improved significantly (p less than .02); exsmokers (n = 15) raised this index from 3.32 to 4.11 l/s. All 9 subjects who abstained from smoking cigarettes for 6 mo and who were re-exposed to 0.4 ppm ozone had significant (p less than .01) reductions in MMF (from a mean of 3.86 +/- 1.32 [standard deviation] to 2.99 +/- 0.94 l/s, i.e., 22.5% reduction) and marginally significant reductions in comfort, as evidenced by an increase in the incidence of dry cough (p less than .10). The FVC was not reduced during ozone exposure. Control exsmokers (n = 6) remained unresponsive to filtered air exposure. The exsmokers' responses to ozone and their improved MMF during the period of smoking abstinence were correlated (coefficient = .88), i.e., subjects with the largest baseline gains in MMF after withdrawal experienced the largest acute decrements in MMF during ozone exposure.(ABSTRACT TRUNCATED AT 250 WORDS)