The use of linear regression for on-line data processing of airway resistance measurements in the body plethysmograph may be difficult. When airflow (V) is plotted against plethysmographic pressure (Pbox), the resulting loop exhibits hysteresis and may drift along the PBox axis. Although the slope of the V-PBox loop can be estimated visually, computer calculations of the slope are difficult if baseline drift is present. Taking the derivatives of V and PBox eliminates the effect of drift and reduces the loops to a series of essentially parallel lines from which the slope can be readily calculated using linear regression. The identical technique can be employed when mouth pressure is plotted against plethysmographic pressure, enabling on-line calculation of thoracic gas volume.