Objective: To evaluate the Datex Deltatrac II for measurements in neonates requiring mechanical ventilation. Design: Prospective laboratory evaluation, using a ventilated lung model and gas injection. During simulation of 79 neonatal respiratory settings, assessment of oxygen consumption (V Ç O 2 ), carbon dioxide production (V Ç CO 2 ) and respiratory quotient (RQ) was compared to a reference method (mass spectrometry, wet gas spirometry) using the statistical method of Bland and Altman. Interventions: Respiratory variables, which may influence the accuracy and precision of gas exchange measurements, were varied within the following ranges: inspired oxygen fraction (F I O 2 ): 0.21±0.8, expired carbon dioxide fraction (F E CO 2 ______ ) and inspiratory-expiratory oxygen fraction (DFO 2 ): 0.0032±0.0256, expiratory flow rate: 1.0±2.5 l/min, inspiratory pressure: 10±55 mbar, respiratory rate 25±60/min, constant RQ of 1. This resulted in 79 tests with V Ç CO 2 and V Ç O 2 ranging from 8±64 ml/min. Measurements and results: The coefficient of repeatability for ten single subsequent Deltatrac measurements was 8.09 ml/min for V Ç O 2 and 9.17 ml/min for V Ç CO 2 compared to 2.02 ml/min and 0.90 ml/min for V Ç O 2 and V Ç CO 2 with repeated reference measurements. The coefficient of repeatability of the Deltatrac measurements improved considerably when means of subsequent 5 min intervals were compared: 0.68 ml/min for V Ç O 2 and 0.28 ml/ min for V Ç CO 2 . The difference between the two methods (Deltatracreference) was ±3.8 % (2 s: 11.4 %) for V Ç O 2 , 13.2 % (2 s: 7.9 %) for V Ç CO 2 and 17.6 % (2 s: 16.7 %) for RQ. The agreement between methods deteriorated with smaller (F E CO 2 ______ ) or DFO 2 and increasing F I O 2 . Conclusions: Considering limits of agreement of less than 20 % as clinically acceptable, results for V Ç O 2 assessment indicate acceptable accuracy and precision whereas V Ç CO 2 and RQ assessments exceed this limit. Limited accuracy and precision result from detection of CO 2 following dilution of expiratory gases and increased sensitivity to error propagation by Haldane equations due to the small differences between inspiratory and expiratory gas fractions.