Ventilatory failure can occur in a variety of conditions and may often go unrecognized clinically. A practical system has been developed for detecting ventilatory failure. The arterial carbon dioxide pressure (Paco2) as determined with this system was compared with the Paco2 as measured directly in 107 tests on 63 patients with heart and lung disease (Paco2 range of 22 to 103 mm Hg). With this system it was possible to make a reasonably accurate and useful estimate of Paco2 (standard error of the estimate, 4.6 mm Hg). The test can be done quickly and the equipment is portable, inexpensive, and suitable for office and bedside use.