Indoor air quality (IAQ) has been understudied in day-care centres (DCCs), even though it can affect the respiratory health of children. This study was undertaken to assess IAQ in a randomly selected sample of 21 DCCs having space for at least 40 children in Montréal, Canada, and to determine associations between building characteristics and IAQ. Questionnaires on building characteristics and operation of the DCC were administered to managers. Temperature, relative humidity, and concentrations of carbon dioxide (CO(2)), formaldehyde and volatile organic compounds were measured in January and February 2008 in rooms attended by children aged between 18 and 60 months. Most DCCs (81%) had a mechanical ventilation system. Over 85% of the DCCs had a mean CO(2) concentration higher than 1000 ppm, the value generally targeted for comfort in buildings. Mean CO(2) concentrations were significantly lower in DCCs having a floor space meeting the provincial standards. The mean (standard deviation-SD) formaldehyde concentration was 22.9 (8.2) µg/m(3), with all participating DCCs being within Health Canada's Residential IAQ Guideline of 50 µg/m(3). The presence of a mechanical ventilation system and a large surface of play area per child were significantly associated with lower CO(2) levels, explaining 44% of the variance in indoor CO(2) concentrations. The presence of a mechanical ventilation system was also associated with significantly lower formaldehyde and acetaldehyde levels. Moreover, 68% of the variance in indoor acetaldehyde concentrations was explained by CO(2) levels, indicating that CO(2) was a better proxy of ventilation than the presence of a ventilation system, as this latter variable did not imply that the ventilation system was running or functioning adequately. These results demonstrate the need for on-going efforts to ensure sufficient floor space and adequate ventilation in DCCs to maintain good IAQ.