A new device for non-invasive monitoring of PCO2, the Hewlett Packard cutaneous capnometer, was studied. Seven patients with disordered breathing, three with sleep apnoea syndrome, and four with chronic respiratory insufficiency, underwent polygraphic sleep recording including non-invasive measurement of oxygen saturation and transcutaneous CO2 pressure (PtcCO2). Two of the apnoea patients showed a modest increase in PtcCO2 with sleep. The patients with respiratory insufficiency showed larger increase in PtcCO2 and more profound hypoxemia during sleep. When the patients with respiratory insufficiency received oxygen (0.3-0.5 l/min) via nasal prongs, the sleep induced hypoxemia almost vanished but their PtcoCO2 increased. Nocturnal hypoventilation probably increased the effectiveness of the low oxygen supply, thus counteracting hypoxemia during periods of hypoventilation. The capnometer was easy to apply and the patients felt no discomfort. It promises to be a useful method for detection of hypercapnia indicating hypoventilation in patients with disordered breathing.