Airtightness, indoor air quality, and air infiltration were measured in ten occupied, detached, two-story houses of wooden construction in the winter of 1986–87. The floor area of the houses was 120 to 160 m2. The houses had various types of heating systems. Seven of the houses had exhaust fan units for ventilating living rooms with air-to-air heat exchangers. Airtightness was measured by the fan pressurization method. Equivalent leakage area per floor area was 1.9 to 5.7 cm2/m2. The concentrations of CO2, NO2, and suspended particles were measured. CO2 and NO2 concentrations in the houses where unvented oil heaters were used were higher than in the other houses. The airtightness and indoor air quality measured during this period were compared with measured results obtained for 13 detached houses during the winter of 1984–5. Air infiltration was measured by the concentration decay method using SF6 as a tracer gas and was compared with the calculated value on the basis of equivalent leakage area.