The aim of our study was to compare four algorithms for the measurement of respiratory resistance in adults using the interrupter technique. Four methods to estimate alveolar pressure from the mouth pressure during occlusion were assessed in 122 normal adults (57 males, 65 females, aged 18-79 yr, mean 41.2 +/- 15.6): taking the mean oscillation pressure (Rint), performing a linear back extrapolation (RintL) or a linear regression for the whole curve (RintRL), taking the end-occlusion pressure (RintE). Mean values for Rint, RintL and RintRL in males were respectively 0.23 +/- 0.04, 0.22 +/- 0.04 and 0.22 +/- 0.04 kPa s L(-1), not statistically different. In females values were respectively 0.27 +/- 0.05, 0.26 +/- 0.05 and 0.26 +/- 0.05 kPa s L(-1), not statistically different. RintE was higher than the remainder and was the only measure correlating weakly with morphometric variables: RintE = 1.152 - (0.00152 x age (yr)) - (0.00382 x height (cm)), r = 0.31 (p < 0.05) in females; RintE = 0.227 - (0.00122 x age (yr)) + (0.00830 x body mass index (kg m(-2))) with r = 0.34 (p < 0.05) in males. We conclude that it is equivalent to estimate alveolar pressure from or extrapolating it to the beginning of occlusion in healthy adults but when taking the end-occlusion pressure, resistance is higher and depends in part on morphometric parameters.