To study the association between ambient air pollutants (AAP) and respiratory symptoms complex (RSC) in preschool children, a cohort of 664 children between the ages of 1 month to 4.5 yr were randomly selected from 28 slums (anganwadi centres) of Lucknow, north India. They were followed up fortnightly for six months. The outcomes assessed were presence of RSC at the time of interview and days on which symptoms had occurred in the past week. Exposure to ambient air sulphur dioxide (SO2), oxides of nitrogen (NOx) and suspended particulate matter (SPM) on the day of the interview or in the week prior, was assessed by ambient air monitoring at 9 centres within the city. The cumulative incidence of RSC was 1.06 and the incidence density per 100 days of follow up was 1.63. All three pollutants were positively correlated with each other and negatively correlated with temperature. Ambient air SPM and SO2 and cooking and heating fuels like dung cakes, wood, coal and kerosene and remaining indoors while the food was cooked were associated with increased incidence of RSC, increased duration of symptoms, or both. We conclude that to improve the respiratory health of preschool children, ambient air SPM and SO2 levels should be kept as low as possible and mothers should be advised to keep children in another room while cooking.