The effect of passive exposure to environmental tobacco smoke in the home on respiratory symptoms and pulmonary function level was studied in a cohort of white children aged 7-11 years examined in six US cities in 1983-1988. For 2,994 children with questionnaire-based exposure data, passive exposure to an additional pack of cigarettes smoked daily in the home was associated with increased incidence of lower respiratory symptoms (odds ratio (OR) = 1.25, 95% confidence interval (CI) 1.10-1.42). For 1,237 children with two consecutive 1-week measurements in both winter and summer, a 30-micrograms/m3 increase in the annual average indoor concentration of respirable particulate matter with an aerodynamic diameter of < 2.5 microns (PM2.5)--that is, approximately the effect of one pack per day of smoking--was marginally associated with an increased cumulative incidence of lower respiratory symptoms (OR = 1.13, 95% CI 0.99-1.30). Indoor measurements of PM2.5 showed no direct association with children's pulmonary function measurements. Children whose mothers smoked during pregnancy showed a reduction of -2.6% (95% CI -5.2% to 0.1%) in volume-adjusted forced expiratory flow rates. Therefore, current indoor exposure to PM2.5 increases the cumulative incidence of lower respiratory symptoms, but is only weakly associated with decreased pulmonary function level in preadolescent children.