One hundred ninety-one simultaneous PtcCO2 and PaCO2 values were recorded from 20 neonates using a heated CO2 sensor. Gestational age and birth weight ranged from 30--44 weeks and 1.22--3.4 kg, respectively. An overestimation of PaCO2 was consistently found. Regression analysis revealed a slope of 1.29, an intercept of 18 mm Hg and a correlation coefficient of 0.85 (p less than 0.001). The PtcCO2 sensor has multiple potential uses in the sick neonate but further experience is necessary to provide adequate correction factors.