Transcutaneous PO2 (tcPO2) has been shown to closely approximate arterial PO2 (PaO2) in infants with normal oxygenation (PaO2 60–100 mm Hg). During hypoxemia (PaO2<60 mm Hg), or administration of vasoactive drugs, such as tolazoline, correlation is frequently so poor that tcPO2 monitoring is of little value. We examined the relationship of tcPO2 to PaO2 among 6 infants with cyanotic congenital heart disease who were receiving prostaglandin E1 (PGE1). Close, linear correlation was found, even during hypoxemia. We conclude that tcPO2 monitoring has potential clinical value in such patients.