A monitoring and research system based on a respiratory mass spectrometer controlled by a microprocessor has been installed in a paediatric intensive care unit. It provides an opportunity to study up to twelve patients simultaneously at distances up to 30 m from the instrument, can operate for prolonged periods without intervention, and includes a routine for regular calibration checks. There is an automatic sensitivity control which compensates for minor variations in probe patency or in water vapour pressure. Information derived from the system can be displayed centrally or at the bedside and data can be conserved for up to 24 hours; past events can be recalled and displayed for either the previous hour or previous 24 hours. The system can be used to measure respired gas tensions, metabolic gas exchange, lung volume, pulmonary blood flow and pulmonary tissue volume. The clinical value and limitations of the system are discussed.