A technique developed to measure functional residual capacity during mechanical ventilation uses an acoustic helium analyzer and microprocessor to perform open-circuit helium washout. The acoustic helium analyzer has acceptable linearity up to a helium concentration of 50% and is stable. The microprocessor combines output from the acoustic analyzer and a ventilator with an expiratory flow signal, to compute lung volume rapidly during mechanical ventilation. In a sample of ten patients with various lung problems, open-circuit helium washout gave results similar to those of a closed-circuit helium-dilution technique during assist control or intermittent mandatory ventilation modes, with or without positive end-expiratory pressure. This open-circuit system has an advantage over other techniques because it is small and portable and can measure lung volume quickly without complicated maneuvers.