We performed counts of airborne spores in a hospital with filtered air where a decrease in nosocomial infection with Aspergillus organisms had been documented. For comparison, similar studies were performed at a nearby general hospital in a ward with open windows. The total spore count inside the hospital with filtered air was significantly less than inside the ward with open windows. The total count of Aspergillus organisms in the filtered air was significantly less than that in the room with open windows. We conclude that the decrease in nosocomial infections with Aspergillus organisms in the hospital with filtered air is probably associated with fewer airborne spores.