We examined the relationship of lung function level to socioeconomic status (SES) using a cross-sectional design among 989 primary school children selected from 18 Montreal schools and studied between April 1990 and November 1992. Information on each child's health, demographics, home exposure to tobacco smoke, pets, and cooking and heating fuel used in the child's home was collected by questionnaire. Spirometry was performed at school. Parental occupation was used to establish SES. After adjusting for personal, familial, and environmental factors, in boys but not in girls, FEV1 and FVC were progressively larger in higher categories of SES (p < 0.001 for linear trend). After taking into account the effect of multiple comparisons and adjusting for personal, familial, and environmental factors, boys from families in the lowest category of SES were found to have an FEV1 lower by 8.2% (95% CI, -13.8 to -2.1) and an FVC lower by 8.1% (95% CI, -13.4 to -2.6) when compared with the most advantaged. These results provide evidence that socioeconomic status, independent of common indoor exposures, is a risk factor for lower FEV1 and FVC among boys.