We evaluated the accuracy of a new device for continuous noninvasive measurement of cutaneous PCO2. The Hewlett-Packard capnometer (model 47210/HA) works by means of an infrared transducer applied to the forearm over an area of skin that has been stripped of the stratum corneum. Capnometer transcutaneous carbon dioxide pressure (CPCO2) was compared with arterial carbon dioxide pressure (PaCO2) during 60 simultaneously obtained measurements in 13 hemodynamically stable patients. Each patient was studied for 1 1/2 to 5 hours, and a wide range of PaCO2 values (21 to 82 mm Hg) was represented. The data show a clinically significant relationship whereby PaCO2 = CPCO2 - 4.13, with a SE of +/- 2.19 mm Hg. Clinical usefulness of noninvasive cutaneous CO2 monitoring can be foreseen in patients whose ventilatory support is being tapered, in those with respiratory depression caused by various neuromuscular disorders, and in patients with chronic obstructive pulmonary disease and acute respiratory failure. Our results indicate that continuous transcutaneous CPCO2 measurements are safe and accurate and strongly suggest that they can be of clinical usefulness in a select group of hemodynamically stable patients.