Pulse oximeters are programmed with a calibration curve derived from studies done in adults. Whether fetal haemoglobin levels affect their reliability is unclear. This study reports the accuracy of pulse oximetry in 22 preterm infants (mean 31 weeks, range 25-36 weeks gestation) between 1 h and 73 days of age. Oxygen saturation obtained from a Nellcor N-200 pulse oximeter (SpO2) was compared with simultaneous arterial values (functional SaO2) measured by a Radiometer OSM3 Hemoximeter over a SpO2 range of 83-99%. Fetal haemoglobin (HbF), carboxyhaemoglobin (HbCO) and methaemoglobin (HbMet) measured by the hemoximeter ranged between 0-100%, 0-3.5% and 0-0.8% respectively. Linear regression analysis revealed a close correlation between SpO2 and functional SaO2 (SpO2 = 0.75 SaO2 + 24.43, r = 0.88, P less than 0.001) over a wide range of values for PCV, heart rate, blood pressure, PaO2, PaCO2 and pH. The mean SpO2-SaO2 difference of 1.3, (s.d. 2.5%, P less than 0.001) was unaffected by HgF, HbCO or HbMet but was increased in infants receiving inotropic support. We conclude that the Nellcor N-200 pulse oximeter gives reliable oxygen saturation measurements unaffected by the HbF level in preterm infants.