A sample of 58 occupied homes in Rochester, New York, most of which incorporated special builder-designed weatherization components, were studied to assess (1) the effectiveness of construction techniques designed to reduce air leakage; (2) the indoor air quality and air-exchange rates in selected tight houses, and (3) the impact on indoor air quality of mechanical ventilation systems employing air-to-air heat exchangers. The specific leakage area was measured in each house using the fan pressurization technique. Houses built with polyethylene vapor barriers and joint-sealing were as a group 50% tighter and had a 30% lower overall average heat loss coefficient (W//sup 0/C-m/sup 2/) than a similar group of houses without such components. Mechanical ventilation systems with air-to-air heat exchangers were installed in nine relatively tight houses, some of which had gas stoves and/or tobacco smoking occupants. Air-exchange rates and indoor concentrations of radon (Rn), formaldehyde (HCHO), nitrogen dioxide (NO/sub 2/), and humidity were measured in each house for one-week periods with and without mechanical ventilation. More detailed measurements including concentrations of carbon monoxide, and inhalable particulates were made in two of these houses by a mobile laboratory. In all nine houses, air-exchange rates were relatively low without mechanical ventilation, 0.2 tomore » 0.5 ach, and yet indoor concentrations of Rn, HCHO, and NO/sub 2/ were below existing guidelines. Mechanical ventilation systems were effective in increasing air-exchange rates and in further reducing indoor contaminant concentrations. The average sensible effectiveness of the heat exchangers was 0.65 +- 0.16. We conclude that when contaminant source strengths are low, acceptable indoor air quality can be compatible with low air-exchange rates. « less