The energy performance for new houses in the Netherlands increased during the last decade with 45%. A shift from Conventional Mechanical Ventilation (natural inlet and mechanical exhaust or CMV) to balanced flow Heat Recovery Ventilation (HRV) can be seen. Several occupants of the newly constructed neighbourhood of Vathorst in Amersfoort reported health complaints that they relate to HRV. The regional Health Board commissioned a study on health complaints and indoor environmental quality in relation to the ventilation system and other building parameters. 100 dwellings were inspected, divided into a case group of complainers in houses with HRV, a group of similar houses without complainers and a control group in dwellings with CMV. Health complaints occurred more often in dwellings with HRV than with CMV. The quality of the indoor environment was inadequate for the parameters noise, draught, CO2, formaldehyde and high indoor temperatures in summer.