['To compare the agreement, precision and repeatability of end tidal carbon dioxide (EtCO2 ) and transcutaneous carbon dioxide (TcCO2 ) with partial pressure of arterial CO(2) ( PaCO2) in postoperative neonates.', 'Fifty mechanically ventilated neonates without lung disease, and with no contraindications for either TcCO2 or EtCO2 monitoring.', 'Paired TcCO2 and EtCO2 values were recorded with three consecutive measurements within the first 48 h of surgery.', 'EtCO2, TcCO2 and PaCO2 triplets were compared using Bland-Altman plots.', 'One hundred thirty-two triplet measures of CO(2) were recorded with mean PaCO2 43.5 (7.3) mm Hg, EtCO2 38.8 (6.4) mm Hg and 43.8 (8.8) mm Hg (p<0.0001 for EtCO2 against PaCO2; paired t test). The PaCO2 - EtCO2 bias±2SD was 4.1±9.0 mm Hg and -0.8±13.0 mm Hg for PaCO2 - TcCO2. 56.1% of EtCO2, and 60.6% of TcCO2 values were within ±5 mm Hg of paired PaCO2.'] Is end tidal carbon dioxide as reliable as transcutaneous monitoring in ventilated postsurgical neonates?