Mechanical ventilation systems have been adopted in airtight energy- efficient houses in Canada to provide fresh air, remove moisture and indoor pollutants and provide a comfortable environment for the home-occupants. Homes constructed under the R-2000 Home Program are equipped with mechanical ventilation with heat recovery. Since 1984, the performance of approximately 700 R-2000 Homes has been monitored on an annual basis. This monitoring has included the measurement of indoor levels of formaldehyde and the documentation of ventilation system operation. Levels of formaldehyde in the initial years of the program averaged 0.069 parts per million (ppm). Formaldehyde originates from urea formaldehyde resin which is used in the manufacture of many household furnishings and building products. After revisions to the R-2000 Home ventilation guidelines which then stipulated fresh air distribution in addition to a total fresh air supply capacity (0.5 ach), the average level of formaldehyde has decreased to 0.045 ppm in the most recent years. Since the decrease of formaldehyde levels with increased age of home was not observed, this decrease has been attributed to the revisions to the ventilation guidelines. Details of the ventilation guidelines, the type of ventilation systems used, and the air quality results are presented.