Particle concentration in patient rooms at various ventilation rates and with differing methods of air supply has been investigated experimentally in a specially built test room. The size of the test room corresponded to an ordinary bedroom for two patients. Air was supplied isothermally to this test room, either through induction units or through a perforated ceiling, and the number of air changes per hour varied from 1 to 16. When the air was supplied by means of induction units, tests with different airflow patterns in the room were also carried out. The principle of the measurements was to estimate the decrease in particle concentration at certain points when the initial concentration was uniform all over the room and no generation took place during the measuring period. Talc powder was used as test dust, and the particle concentration was measured at up to six points in the room with a particle counter.