To find out whether exposure to sulphur dioxide during infancy is related to the prevalence of bronchial hyper-responsiveness (BHR), we studied schoolchildren (aged 7-13 years) from two areas of Norway--a valley containing a sulphur-dioxide-emitting aluminium smelter and a similar but non-industrialised valley. Bronchial responsiveness was assessed in 529 of the 620 participants. The median exposures to sulphur dioxide and fluoride were 37.1 micrograms/m3 and 4.4 micrograms/m3 at ages 0-12 months and 37.9 micrograms/m3 and 4.4 micrograms/m3 at 13-36 months. The risk of BHR increased with exposure to sulphur dioxide and fluoride at these ages; the odds ratio for a 10 micrograms/m3 increase in sulphur dioxide exposure at 0-12 months was 1.62 (95% CI 1.11-2.35) and that for a 1 microgram/m3 increase in fluoride exposure was 1.35 (1.07-1.70) at 0-12 months and 1.38 (1.05-1.82) at 13-36 months. Exposure to these low concentrations of airway irritants during early childhood is associated with an increased prevalence of BHR in schoolchildren.