Non-invasive measurements of oxygen saturation are attractive because they provide continuous information which may result in improved patient care. We evaluated a new finger pulse oximeter as a measure of arterial oxygen saturation in critically ill patients with respiratory distress. In 12 patients studied, 133 simultaneous comparisons between in vivo (Pulse-Oximeter, Nellcor, Hayward, California) and in vitro (IL CO-Oximeter 282) oxygen-haemoglobin saturation were made. Linear regression analysis of these pooled data yielded on excellent correlation (r = 0.97, p less than 0.001, 0.3% accuracy). The results demonstrate that non-invasive oxygen monitoring of patients with respiratory failure is feasible and can reliably detect potential life-threating arterial oxygen desaturation.