Tracheal and arterial CO2partial pressures were measured simultaneously in 27 laryngectomized patients both while they were awake and during high-frequency jet ventilation. Tracheal gas was sampled during brief interruptions of high-frequency jet ventilation. Agreement between tracheal and arterial CO2partial pressures was assessed using the Bland-Altman method. The tracheal-arterial CO2partial pressures gradient during spontaneous breathing was significantly lower (P<0.0002) than during high-frequency jet ventilation. During spontaneous ventilation, the bias was −0.77 kPa (95% CI= −0.99 to −0.55 kPa), and the upper and lower limits of agreement were 0.29 kPa (95% CI = −0.11 to −0.7 kPa) and −1.83 kPa (95% CI = −2.24 to −1.43 kPa). During high-frequency jet ventilation, the bias was −1.61 kPa (95% CI = −1.76 to −1.46 kPa), and the limits of agreement were −0.48 kPa (95% CI = −0.75 to −0.21 kPa) and −2.74 kPa (95% CI = −3.01 to −2.47 kPa). Despite the poor agreement between tracheal CO2partial pressure and arterial CO2partial pressure, it is sufficient to allow for adjustment of ventilator settings during jet ventilation.