The level of cleanliness in the laminar airflow room of Tokai University Hospital was studied. Bacteriological monitoring of the protected isolation ward during occupancy by patients revealed that it was kept at class 100 according to NASA standards. The laminar airflow room was decontaminated with formaldehyde gas by means of a gas generator which we made. This decontamination method was shown to be easy and safe to handle, and stable in its effects. Up to December 1981, 54 patients with acute leukemia were treated in the protected isolation ward. The incidence of infection in patients with acute leukemia was lower in the protected isolation ward than in the conventional ward. No respiratory infection was observed in the protected isolation ward. These results demonstrated statistically significant advantages of the use of the laminar airflow room for prevention of infection.