The concentrations of ultra fine particles (UFPs) were measured in the medium-size city ofGothenburg, Sweden, in the large city of Copenhagen and at a rural site in Denmark. InGothenburg, field measurements were conducted both in several residential and officebuildings, while in Denmark measurements comprise two office buildings, one of themlocated at a rural site. Concentrations of UFPs were measured simultaneously indoors andoutdoors. The results revealed that outdoor levels are major contributors to the indoor particlenumber concentration and the variability in concentrations is less pronounced indoors whenno indoor sources are present. The magnitude of UFP concentrations is higher in the large citycompared to the medium-size city. The results showed that in the Gothenburg office buildingsthe UFP concentrations indoors were fairly correlated to that outdoors. Another differencebetween Danish and Swedish offices is that in Denmark tobacco smoking is a main indoorsource of UFPs. The results from a Swedish residential building show that the indoorconcentration was strongly influenced by the indoor activity, e.g. cooking, ironing and byoutdoor levels mainly during window airing.