Respiratory symptoms and pulmonary function were compared in 46 nonsmoking children aged 10 to 16 years, whose parents were smokers, and an identical number of children (matched for sex, age and height) whose parents were nonsmokers. Passive exposure to parental tobacco smoke resulted in a higher prevalence of respiratory symptoms, more frequent upper airway infections and a significant decrease in forced expiratory flows; these effects were more marked in boys. The single-breath nitrogen washout test, a sensitive test of small airways obstruction in adults, did not detect any effect of involuntary smoking in this limited sample of children.