One hundred ninety-one simultaneous PtcCO2and PaCO2values were recorded from 20 neonates using a heated CO2sensor. Gestational age and birth weight ranged from 30–44 weeks and 1.22–3.4 kg, respectively. An overestimation of PaCO2was consistently found. Regression analysis revealed a slope of 1.29, an intercept of 18 mm Hg and a correlation coefficient of 0.85 (p< 0.001). The PtcCO2sensor has multiple potential uses in the sick neonate but further experience is necessary to provide adequate correction factors.