Despite the well-documented correlation between transcutaneous and arterial PO2 and PCO2 in sick neonates, the effect of maturation on this relationship has not been well characterized. Eight premature infants with bronchopulmonary dysplasia (BPD) and indwelling arterial lines beyond the immediate neonatal period were studied. Transcutaneous PO2 always underestimated PaO2 beyond 10 weeks of postnatal life, such that transcutaneous PO2 - PaO2 was -16 +/- 5 torr (P less than .001). Corrected transcutaneous PCO2 simultaneously overestimated PaCO2 by 9 +/- 3 torr (P less than .001), although this occurred over a wider range of postnatal ages. Transcutaneous PO2 monitoring may be a useful tool for estimating PaO2 in this population, provided an appropriate correction is made beyond 10 weeks of age. It is suggested that caution be exercised when using transcutaneous PCO2 measurements to estimate absolute arterial values in older infants with bronchopulmonary dysplasia.