Measurement of functional residual capacity (FRC) by the open-circuit multiple breath tracer gas washout technique is an established method. A system based upon washout of sulfur hexafluoride (SF6) during mechanical ventilation is described. The central unit in the system is a sensitive and rapid-response infrared SF6analyzer. SF6is washed in until the alveolar concentration of SF6is 0.5%, a concentration so low that the supply of other gases is hardly influenced. During washout, the flow of SF6from the lungs is calculated by a computer every 10 ms from signals representing expiratory flow and SF6concentration. The total volume of SF6, washed out, is calculated by integration of SF6flow. Since the alveolar concentration at the end of washin is known, the lung volume may be obtained. The measurement procedure is highly automated and the result is presented by the computer immediately after washout. Accurate and reproducible results in model lung tests were obtained during air and N2O/O2ventilation. Comparison with body plethysmography (FRCBOX) in eight sitting healthy subjects gave the following: FRCN2= 7 ml + 0.98 × FRCBOX, r = 0.99. Comparison with nitrogen washout (FRCN2) in five postoperative patients gave the following: FRCSF6= 59 ml + 0.97 × FRCN2, r = 0.97. FRCSF6, during N2O/O2ventilation was the same as during air/O2ventilation in a group of paralyzed patients. The measurement system has not been tested in patients with obstructive lung disease.