In the study, the factors affecting concentrations of radon vertically lines were surveyed in twolarge office buildings. Integrated concentrations of radon were determined with alpha track etchfilms (2 months) and continuous monitoring (2-6 days) was carried out with Pylon AB-5equipment. The effective air exchange rates were analysed by the tracer gas method with aninfrared analyser and rates of air flows from vents were measured with a thermoanemometer.Pressure differences were measured with a manometer and temperature differences withthermoelements. Measurements were conducted during springtime 1997.Continuously measured levels of radon varied from 17 to 129 Bq m-3. The variation of integratedradon levels including nights and weekends was larger ranging from 20 to 350 Bq m-3. TheFinnish limit value of 400 Bq m-3 was not exceeded in any room. Correlation betweencontinuously measured and integrated concentrations of radon was quite high (R2=0.8125).Concentration of radon was observed to increase if depressurisation of the room increased, theeffect was seen even on the third floor. Expectedly, radon levels were the highest in piping tunnelsnearest the soil. However, there were not essential changes in radon levels between different floors.Radon levels correlated with air exchange rate quite good in building A (R2=0.4809) and onlyslightly in building B (R2=0.0542). Theoretical concentrations of radon due to stone buildingmaterial were generally less than 50 % of measured concentration.