Objectives To compare the agreement, precision and repeatability of end tidal carbon dioxide () and transcutaneous carbon dioxide () with partial pressure of arterial CO₂ () in postoperative neonates.

Patients Fifty mechanically ventilated neonates without lung disease, and with no contraindications for either or monitoring.

Interventions Paired and values were recorded with three consecutive measurements within the first 48 h of surgery.

Main outcome measures , and triplets were compared using Bland-Altman plots.

Results One hundred thirty-two triplet measures of CO₂ were recorded with mean 43.5 (7.3) mm Hg, 38.8 (6.4) mm Hg and 43.8 (8.8) mm Hg (p<0.0001 for against ; paired t test). The − bias±2SD was 4.1±9.0 mm Hg and −0.8±13.0 mm Hg for −. 56.1% of , and 60.6% of values were within ±5 mm Hg of paired .

Conclusions In postoperative neonates, and demonstrated a clinically acceptable agreement with .