The indoor air quality and occupant satisfaction was studied in five mechanically ventilated and four naturally ventilated open-plan office buildings in Copenhagen, using a simplified assessment procedure. Temperature and the concentration of CO2 were monitored in 2-10 locations per office to evaluate the variation throughout the offices. A representative measurement point was subsequently selected and measurements of the same parameters were made during one week. All offices were monitored during the same week and occupant responses to the indoor environment were collected via the Internet on the same day within that week. The study indicated that occupants in naturally ventilated offices have a lower prevalence of symptoms than those in mechanically ventilated offices. Although the room air temperature varied more and the concentration of CO2 was higher in the naturally ventilated offices the results showed that the occupants' satisfaction with the indoor environment was higher in naturally ventilated than in mechanically ventilated offices.