The results of pulmonary diffusing capacity (DLCO) measurements by the steady state (DLCOss) and single breath (DLCOsb) methods were compared in 50 normal subjects (Group A), 50 patients with bronchial asthma (Group B) and 50 patients with chronic obstructive lung disease (COAD) (Group C). A significant correlation was observed between DLCOss and DLCOsb in all the three groups of subjects. This relationship was stronger in Groups A and B as compared to Group C. Both DLCOsb and DLCOss were significantly correlated to parameters of air-flow obstruction. Both measurements of DLCO appear valid in normal subjects and in patients with bronchial asthma. In patients with COAD, on the other hand, DLCOsb may be valid measurement, since it is less likely to be influenced by airways obstruction.