When using a nasal cannula to sample gases expired by a patient, air from the room may dilute the sample. For this reason, the accuracy of the partial pressure of end-tidal carbon dioxide (ETCO2) measurements is questionable. We experimentally examined the reliability of ETCO2 measurements through a nasal cannula and found that they depended on both biological factors (tidal volume and respiratory rates) and mechanical factors (the diameter and the length of the cannula and the diameter of the prongs). These results suggest that the correct use of an appropriate sampling cannula will provide reliable ETCO2 measurements without clinical problems.