This study of paediatric intensive care patients aimed to determine where pulse oximetry probes shouid be placed to obtain the most accurate and reliable readings of peripheral oxygen saturation (SpO2). Using arterial blood gas analysis (SaO2) as the gold standard and SpO2 92% and SaO2 < 90% as indicators of hypoxaemia, negative predictive values of SpO2 were 96%, 98% and 98% at the ear, thumb and big toe respectively in 110 children, and 93% at all 3 sites in 90 neonates. The highest clinical agreement between SaG, and SpO2 was for ear probes in children (kappa = 0.70) and the lowest was for big toe probes (kappa = 0.57 and 0.28 in children and neonates respectively).