To prevent nosocomial pulmonary aspergillosis during hospital construction, neutropenic patients with hematologic malignancy were required to wear high-efficiency masks when leaving their rooms. The rate of nosocomial aspergillosis decreased from 0.73 per 1,000 hospital patient-days during fiscal years 1993 to 1996 to 0.24 per 1,000 hospital patient-days during fiscal years 1996 to 1999 (P < .001). High-efficiency masks reduced nosocomial aspergillosis during hospital construction.