We investigated the effects of occasional exposure to environmental tobacco smoke (ETS) on lung function in children. A total of 317 healthy 12- to 15-yr-old nonsmoking children and adolescents, living in households in which none of the adults were active smokers, were selected for the present analysis. The urinary cotinine:creatinine ratio (CCR) was taken as the biologic indicator of exposure, and children were classified according to CCR quartiles. The ratio between FEV1 and FVC (FEV1/FVC) and both early (peak expiratory flow [PEF]) and midexpiratory flow rates (FEF25-75) were lower in children with higher CCR values. In percent terms, the decrease in adjusted lung function for children belonging to the second, third, and fourth quartiles in comparison with those in the first quartile was -1.37, -2.12, and -1.94 for FEV1/FVC (p for trend = 0.012); 0.98, -3.31, and -4.54 for PEF (p for trend = 0.024); and -0.05, -4.85, and -6.13 for FEF25-75 (p for trend = 0.022), respectively. The effects remained significant when possible confounding by father's education and urinary creatinine level had been taken into account, and when children whose parents' smoking status may have been misclassified were excluded. This study suggests an adverse effect on lung function from even low-level exposure to ETS among nonsmoking children living with reportedly nonsmoking parents.