It is estimated that about 4 million flats in Poland are made of prefabricated elements in different systems. Moreover, at present more than 10 million Poles live in system buildings. Despite the commonness of those buildings the analyses of the microclimate of the interior space are very rare. The paper presents the results of the thermal comfort measurements conducted in different flats of multi-family panel building made in W70 system. It appears there is a problem with the overheating effect during the summer months in all parts of the building.