Buildings with indoor air quality (IAQ) complaints frequently have high airborne concentrations of Penicillium species, while buildings with few IAQ complaints have an indoor air (IDA) fungal ecology similar to outdoor air (ODA), where Cladosporium species is usually the dominant microorganism. These studies compared fungal air profiles, measured continually over 6 h in a documented sick building, in IDA in a room experiencing IAQ problems with fungal profiles measured concurrently in ODA. The dominant species collected at both sites were Penicillium species, Cladosporium species, and Alternaria species. In the IDA, Penicillium species were always the dominant organisms, ranging from 150 to 567 cfu/m3 (89.8-100% of the total fungi). In the ODA, Cladosporium species were dominant in four samples (40.0-70.6%), while Penicillium species were dominant (52.7-79.6%) in two. These data demonstrate that, even though ODA fungal profiles are changing continuously, IDA fungal profiles in "sick" buildings tend to remain unchanged.