We studied the frequency of changes in maximal mid-expiratory flow rate (MMF) in 32 nonsmokers and 22 smokers with diffuse interstitial lung disease (DILD) and in 36 health nonsmoking control subjects. Supernormal MMF to total lung capacity (TLC) and MMF to forced vital capacity (FVC) ratios (greater than 120% of predicted) were significantly more common in nonsmokers with DILD than in healthy nonsmokers, but the prevalence of supernormal forced expired volume in 1 s (FEV1) to FVC ratios was similar. In non-smokers with DILD, a supernormal MF/TLC ratio was associated with a significantly greater static lung recoil pressure (Pst) and a tendency toward a higher maximal expiratory flow to Pst ratio than in normals, suggesting that the increased MMF is due to the combined effects of increased driving pressure for flow and reduced resistance in peripheral airways.