Arterialized ear lobe blood samples have been described as adequate to gauge gas exchange in acute and chronically ill patients. It is a safe procedure, usually performed by medical technicians. We have conducted a prospective study to verify the validity of this method. One hundred and fifteen consecutive adult patients were studied. Blood samples were drawn simultaneously from arterialized earlobe and radial artery. Values of partial pressure of oxygen (PO2) and of carbon dioxide (PCO2) were measured by means of blood gas electrodes. The correlation coefficients between the two samples were 0.928 for PO2 and 0.957 for PCO2 values. In spite of a highly significant correlation, the limits of agreement between the two methods were wide for PO2. Earlobe values of PO2 were usually lower than arterial values, with larger differences in the range of normal arterial PO2. On the other hand, the error and the limits of agreement were smaller for PCO2. We conclude that, in adult patients, arterialized earlobe blood PO2 is not a reliable mirror of arterial PO2.