The health and comfort of building users is an important aspect in determining their quality of life. An increase in particulate matter (PM) air contamination and its negative impact on human health have resulted in efforts to monitor and identify its sources. The occurrence of indoor PM depends on outdoor pollution and its transportation indoors as well as the presence of indoor pollution sources. Indoor sources include building materials; furnishing, heating, ventilation and air conditioning systems; activities related to combustion processes; and occupancy activities. This paper aims to carry out an experimental investigation of PM in the indoor air of selected offices. The mass and number concentrations of PM for fractions of 0.5, 1.0, 2.5, 5.0 and 10.0 mu m were monitored and evaluated to determine the extent of exposure of employees in office buildings in Slovakia. The monitoring of eight offices showed that mass concentrations of PM10 exceeded the maximum allowed value in 37.5% of the monitored offices. PM mass concentrations ranged widely based on particle size: 28.0-113.4, 19.0-59.5, 4.6-17.0, 1.9-11.4 and 1.1-7.0 mu g.m(-3) for PM10, PM5, PM2.5, PM1 and PM0.5, respectively. Measurement results for two offices when they were occupied and unoccupied showed higher particle mass concentrations in unoccupied offices than in occupied offices for all size fractions. Measurements of number concentrations of 0.3-1.0 mu m fractions were higher in offices without activity than concentrations in offices with activity.