The accuracy and reliability of the relationship between arterial and end-tidal carbon dioxide (Petco2), expressed as the gradient, P(a - et)CO2, was assessed with 171 comparisons in nine mechanically ventialted trauma patients. The P(a - et)CO2, was 14 ± 11 mm Hg. (mean ± standard deviation). The positive correlation between Paco2(44 ± 10 mm Hg) and Petco2(30 ± 10 mm Hg) for the study population (reflected by r = 0.64, p = 0.001; but r2= 0.41) indicated statistical significance, but only 40% of the changes reflected a linear relationship. Seventyeight percent of individual patients had significant correlations of Paco2and Petco2(p > 0.02 to p > 0.001). Changes in Petco2erroneously predicted the Paco2changes in 27% of comparisons with 15% false decreases and 12% false increses. Trends in P(a - et)co2magnitude are not reliable concordant direction changes in Petco2and Paco2are not assured.