Describes the microbioligic surveillance of the central air handling systems in a hospital, prompted by an increase in the rates of nosocomial infection caused by filamentous fungi in immunocompromised patients. Heavy contamination by Penicillium was found in ventilation ducts and terminal unit lined with fiberglass in the operating room air handling system, as well as Aspergillus species. The contamination was corrected using filter replacement and decontamination with aerosolized chlorine solution. Contamination recurred after seven months and it was necessary to perform the procedure again. Recommends certification guidelines for hospital ventilation systems.