Discusses sick building syndrome in office buildings. Poor ventilation is frequently blamed, although in 60% of cases, no specific cause can be identified, because of symptoms which are non-specific, and the vast variety of substances present in the air. Similarly, factors contributing to disease are not restricted to pollutant levels only. Procedure for examining causes of sick building syndrome are suggested. Describes checking of different kinds of ventilation system, and the importance of outside air intake. Describes problems of both local and central ventilation systems.