The accuracy with which transcutaneous measurements of oxygen tension reflect PaO2 in older infants has recently been questioned. We therefore examined the effect of maturation, i.e., age or skinfold thickness, on the accuracy of transcutaneous oxygen tension (PtcO2) and oxygen saturation (StcO2) measurements in 19 infants (age 1 to 61 wk) undergoing elective cardiac catheterization. Twenty-seven simultaneous arterial and transcutaneous measurements revealed a good correlation between PtcO2 and PaO2 (r = .91, slope .77, intercept 3.23 torr). The mean arterial-transcutaneous PO2 difference of 10 torr (range - 15 to 35) was independent of age but was significantly correlated with skinfold thickness (r = .45, p less than .05). There was also a good correlation between StcO2 and SaO2 (r = .95, slope .65, intercept 27.8%). The mean arterial-transcutaneous oxygen saturation of 1.4% (range - 17.3 to 14) was unaffected by age or skinfold thickness. However, neither PtcO2 or StcO2 measurements were accurate in patients with severe hypoxemia; StcO2 consistently overestimated the SaO2 when the SaO2 was below 70%. Thus, in this study the discrepant PtcO2 measurements in older infants were due to increasing skinfold thickness rather than age. PtcO2 monitoring still has an important role in oxygen monitoring and together with StcO2 provides valuable information on oxygenation.