Tracheal and arterial CO2 partial 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 CO2 partial pressures was assessed using the Bland-Altman method. The tracheal-arterial CO2 partial 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 CO2 partial pressure and arterial CO2 partial pressure, it is sufficient to allow for adjustment of ventilator settings during jet ventilation.