In this study, the effect of duct cleaning on the indoor air quality and on the ventilation system hygiene in non-problem office buildings: was investigated. Six office buildings were selected from local duct cleaning companies' contracts. A questionary was distributed among the occupants prior to and after the duct cleaning. Also several air quality parameters were measured in outdoor air, indoor air,· and supply air and ventilation system hygiene measurements were performed prior to and after duct cleaning. Based on the results, the effect of duct cleaning on indoor air quality and ventilation system hygiene was estimated. The indoor air quality and the amount of complaints and symptoms were about the same before and after the duct cleaning, however, dust concentrations on the inner duct surfaces decreased in almost every building.