The phase difference between mouth flow and chest flow in cases of pulmonary diseases measured using a body box was compared with those in normal subjects. Phase differences in chronic pulmonary emphysema (CPE) and diffuse panbronchiolitis (DPB) patients increased compared to normal subjects, but remained normal in interstitial pneumonitis patients (Normal: 3.76 +/- 1.71, CPE: 10.70 +/- 4.93, DPB: 10.81 +/- 3.15, IP: 3.83 +/- 0.15 degrees). In addition, there was a good correlation with FEV1.0, FEV1.0% and RV/TLC (r: -0.634, -0.610 and 0.803). The analysis of phase differences during rest breathing is clinically useful because it is non-effort dependent and can evaluate the degree of airway disorders.