The phenomenon of 'sick building syndrome' is associated with several factors rather than a single readily and directly observable cause and various symptoms and behaviours rather than one clinical syndrome. Like another modern occupational ill health problem--repetitive strain injury--its existence may be questioned or doubted by the employers of staff who work in modern buildings. This article looks at what makes a building sick and how a sick building can affect its occupants. It guides staff in the recognition of the syndrome and suggests ways to reduce the risks of working in a building whose poor design or maintenance constitutes a hazard for occupants.