Previous reports found that the conventional single breath diffusing capacity for carbon monoxide (DLcoSb) was reduced in smokers and correlated with macroscopic disease, as detected by CT scan techniques . However, in seated patients at rest DLcoSb was insensitive to the interregional nonuniformity in the distribution of emphysema that often preferentially affects the apex of the lung in smokers . Conventional DLcoSb testing, as distinct from the DLcoSb-3EQ , was also less useful as a screening tool because the method was spuriously affected by the variations in the way the single breath test was performed . Interpretation of DLcoSb in smokers has also been potentially confounded by uncertainties about precise corrections of DLcoSb for carbon monoxide back pressure, carboxy-hemoglobin and the binding affinity of hemoglobin . In the present study we corrected all measurements of DLcoSb-3EQ for the carbon monoxide back pressure by measuring the carbon monoxide in the exhaled gas during the exhalation phase of the deep breath of room air preceding each DLcoSb-3EQ manoeuvre . The infrared carbon monoxide analyzer was sufficiently sensitive to allow measurement of background carbon monoxide in nonsmokers at levels of 0 to 70 ppm . The carbon monoxide analyzer was linear in this range and was checked daily .