OBJECTIVE To investigate an unexpectedly high initial skin CO2 pressure with a new small earlobe probe* heated to 42 degrees C containing both transcutaneous (tcPCO2) and pulse oximeter saturation (SpO2) sensors. METHODS The probe was mounted on the ear lobe of six patients during abdominal or thoracic surgery and on several awake volunteers. The probe was mounted on a cheek or forearm in two other volunteers. Patients were artificially ventilated under general anesthesia at constant end-expiratory PCO2. RESULTS In patients, at 8 +/- 3 min after mounting, tcPCO2 peaked 5 mmHg higher than its final value (p = 0.0067, n = 6, paired t-test). After 25 min, tcPCO2 was not different from PaCO2 (arterial). Similar overshoots were recorded with this device when mounted on the arm or cheek and with a standard transcutaneous PCO2 probe set to 42 degrees C, mounted on the ear lobe, arm or chest of awake volunteers. In two volunteers, we found that heating the sensor to 45 degrees C for the first 15 min on the ear, and then decreasing it to 42 degrees C prevented overshoot, and provided valid tcPCO2 data 3 - 5 min after application of the sensor. CONCLUSIONS A temperature of 42 degrees C may increase local skin temperature and metabolism before vasodilating more remote arteriolar control of sub-sensor capillary flow. We suggest that transcutaneous PCO2 probes be initially set to 44 - 45 degrees C for 5 - 15 min to induce prompt vasodilation to prevent this overshoot and then reduced to 42 degrees C to avoid skin thermal injury in case of long-term application.