Air filters of various types, selected on the basis of discoloration, were collected from the primary and secondary filter banks of the heating, ventilating, and air-conditioning systems in seven hospitals in the eastern United States and examined with direct microscopy for fungal colonization. Microscopic observations and culture results showed that filters from five of the hospitals were colonized with fungi including species of Acremonium, Alternaria, Aspergillus, Cladosporium, Epicoccum, Penicillium, and Rhinocladiella, and a Beauveria-like fungus. Several of these commonly airborne species, e.g., Epicoccum purpurescens (syn. E. nigrum) and Rhinocladiella sp., had not been previously reported to colonize (with conidiogenesis) air filters.