Reducing the concentration of filamentous fungal spores in the air inhaled by immunosuppressed patients is an important goal. Although it is not clear that air is the exclusive vector of nosocomial filamentous fungal disease, it is quite likely that it is the most important vector. Uncertainties about this assertion are compounded by the great variety of fungal species involved. Aspergillus fumigatus, A. flavus, A. terreus, A. niger, Petriellidium boydii, Fusarium sp., Mucoraceae, Phoma sp., Alternaria, Penicillium, and others have caused invasive disease in immunosuppressed patients. But detailed studies of environmental correlates of nosocomial disease are largely restricted to A. flavus and A. fumigatus. Spores of all of these species may be recovered from unfiltered air. It is probably reasonable to extrapolate from Aspergillus to all of the species involved.