An ultrasonic flowmeter could be advantageous over a differential pressure pneumotachograph having a constant error in varying conditions, The irt vitro accuracy of ultrasonic tidal volume (V-T) estimates for ventilated infants were evaluated.Flow linearity and frequency response were tested, as was the influence of humidity and oxygen content on the accuracy of V-T estimates.The linearity was within the 5% limits between -350 and 350 mL.sec(-1) and was not affected by the presence of an endotracheal tube (ET), The frequency response was flat and unaffected by an ET up to 4.5 Hz, The V-T in the range 7-100 mL, in air showed a mean error of 0.1% (95% confidence interval (CI) -0.2-0.4%) with a maximum and minimum of 6.5 and -3.5% respectively, Humidity did not affect accuracy. After calibration in air, the maximal mean error for measurements in pure oxygen was 3.0% (95% CI 1.9-4.1%). Repeated measurements over 5.5 h had a mean error of 0.4% (95% CI -0.7-0.1%).The in vitro evaluation of an ultrasonic flowmeter showed stable accuracy In mechanical ventilation conditions. Changing connection geometry and oxygen content did not increase the error to a clinically relevant degree. The flowmeter could therefore be a better alternative than the pneumotachograph for ventilated infants.