The ventilation rates of 23 operating theatres were tested randomly. These were found to vary widely from design specifications (--43 to +40%). One modern theatre was studied intensively for 6 months and the causes of poor ventilation determined. Preventative maintenance schemes are justified by a positive relationship with plant performance. The influence of theatre ventilation of contamination with anaesthetic agents and the medico-legal implications of poor theatre ventilation are discussed. There is a need for main duct airflow signals, displayed in theatre, to warn personnel of low levels of theatre ventilation.