Forty-one homes of conventional design were studied to investigate the relationship between bioaerosols, building parameters, and season, and to determine if differences existed across health-based home categories. The homes were categorized as: those for which no indoor air problems were known (noncomplaint homes), noncomplaint homes of allergy patients (intervention homes), and complaint “sick” homes (complaint homes). Carbon dioxide and relative humidity were measured in the basement and main floor areas. CO₂ concentrations were elevated for complaint homes (mean 1190 ppm) but less than 1000 ppm for all noncomplaint homes (mean 550 ppm). Relative humidity was significantly lower for intervention homes than for complaint or noncomplaint homes. Viable and nonviable bioaerosol sampling was performed on the main floor, the basement, and outside. Outdoor viable fungi exhibited an 8.4-fold range when plotted by month, but respirable and nonrespirable indoor fungal concentrations did not differ significantly by season. Basement geometric mean concentrations of fungi exceeded twice outdoor levels for complaint homes but were half the outdoor concentration for noncomplaint homes. Analysis of variance of bioaerosol concentrations revealed higher contamination in complaint than in noncomplaint homes, and concentrations in intervention homes were significantly lower than the other two groups. Overall, 80% of total viable fungi and 55% of bacteria were respirable. The predominant genera were Cladosporium in noncomplaint and intervention homes, while Penicillium and Aspergillus dominated in basements of complaint homes. The presence of pets, having an unfinished basement, absence of central air conditioning, and decreased use of air conditioning were significantly associated with elevated levels of fungi. High-efficiency furnace filters and increased use of central air conditioning contributed significantly to lower fungal levels in intervention homes.