This study evaluates whether the pulse oximeter is useful during cardiopulmonary resuscitation. The instrument was of undoubted benefit in the management of primary respiratory arrest, but of equivocable value in cardiac arrest. Data were displayed during external chest compressions, but were generally unreliable. The ear probe was not able to produce a consistently good signal during cardiopulmonary resuscitation. Nevertheless, the provision of an oximeter amongst the first-line resuscitation equipment available at a cardiopulmonary arrest significantly altered the management of seven out of 20 patients, five of whom survived.