An increase in the recovery of Aspergillus flavus from respiratory specimens occurred among hospitalized patients during a several-month period in 1977. Notably, 86% of the patients with positive culture results were located in an older hospital section adjacent to a building construction site that created great amounts of dust in the hospital's vicinity. Airborne contamination was suspected and results of air sampling showed A. flavus in 80% of old wing patient rooms (average, 8 A. flavus/positive room) compared with only 23% of the contiguous newer wing patient rooms (average, 1 A. flavus/positive room). Inspection of the main mechanical ventilation units in the two hospital sections disclosed numerous defects in the unit in the old wing and a properly functioning unit in the new wing. After repairing the defective unit, a significant reduction in the recovery of A. flavus from clinical and environmental specimens was noted.