Despite the well-documented correlation between transcutaneous and arterial P(O2) and P(CO2) 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 P(O2) always underestimated Pa(O2) beyond 10 weeks of postnatal life, such that transcutaneous P(O2) - Pa(O2) was -16 ± 5 torr (P < .001). Corrected transcutaneous P(CO2) simultaneously overestimated Pa(CO2) by 9 ± 3 torr (P < .001), although this occurred over a wider range of postnatal ages. Transcutaneous P(O2) monitoring may be a useful tool for estimating Pa(O2) in this population, provided an appropriate correction is made beyond 10 weeks of age. It is suggested that caution be exercised when using transcutaneous P(CO2) measurements to estimate absolute arterial values in older infants with bronchopulmonary dysplasia.