Building-related illness is an increasingly common problem. The disease fall into two categories : those that have an identifiable cause-such as legionellosis, humidifier fever, and conditions resulting from exposure to known substances such as asbestos, lead in paint, formaldehyde, etc-and those that have no readily identifiable cause but can be described only by a group of symptoms known as sick building syndrome (SBS). Although objective physiologic abnormalities are generally not found and permanent sequelae are rare, the symptoms of SBS can be uncomfortable, even disabling, and whole workplaces may be rendered non-functional. In assessment of patients with SBS complaints, specific building-related illnesses should be ruled out by history or physical examination. On-site assessment of buildings is extremely useful. Symptoms of non-specific building-related illnesses are common ; their heterogeneity suggests that they do not represent a single disorder. Although there is little convincing, direct evidence to implicate specific causative agents, there is sufficient indirect evidence to support a number of recommendations. For example, it seems prudent to maintain an outdoor-air supply of more than 10 liters per second per person ; to select the building materials, furnishings, and equipments that are least likely to release pollutants such as formaldehyde or volatile organic compounds ; to ensure proper maintenance and cleaning ; and to avoid materials that may act as substrates for the proliferation of microbes or dust mites.