One hundred and thirty-seven individuals residing in 59 defective homes in San Diego County complained of chronic and recurrent symptoms between 1984 and 1997. Symptoms included fungal infections of the skin, ears, sinuses, and nails; sinusitis, otitis media and externa, rhinoconjunctivitis, asthma, and bronchitis, as well as chronic cough, shortness of breath, and chronic fatigue. The association of the symptoms with the homes was strengthened by almost universal, although independent, noting of significant improvement in non-infectious symptoms when away from home for one or more days. Studies of selected homes revealed up to 15 separate species of fungi (or molds) with mold spore counts up to 15,000 spores per cubic meter of air. Destructive testing, tape samples, gravity plates, and Anderson traps revealed the types and sources of mold growth due to excessive water intrusion through the roofs, window frames, walls, and concrete slabs. Mold was found in the carpets, pads, furniture, clothing, walls, closets, and even inside the walls and behind the tiles in the showers and bathrooms. Return air ducts beneath the floors in some of the homes were noted to have standing water in them and many hot air ducts were laden with molds thereby distributing mold spores to every room in the house. As indicated in a survey of ten years of peer-reviewed literature, one or more positive associations were found between fungal levels inside the homes and health outcomes. There are no accepted guidelines for safe levels of fungi in home environments based on health risk assessment. Tests to reveal the presence of mold toxins and the consequences of specific levels of airborne mold spores on human health are in urgent need of development. Studies designed to quantitate the role of fungi in diseases due to“sick building syndrome” should address factors such as susceptible populations, atopic sensitization to fungi, responsiveness to inflammatory agents and irritants, quantitative measure of fungal particles, and the role of co-pollutants such as allergens (dust mites, cockroach, danders, & pollens), inflammatory agents such as exotoxins and endotoxins, and irritants such as ozone, nitrogen oxides, and volatiles.