In order to assess the additive effects of taking into account dead space volume (VD), carbon dioxide, hemoglobin (Hb) and carboxyhemoglobin on computation of single breath carbon monoxide diffusing capacity (DLCOsb), we sequentially applied all the corrections recommended by the 1987 American Thoracic Society (ATS) document on DLCOsb standardization. We used data from 739 men (333 nonsmokers and 406 current smokers) and 475 women (403 nonsmokers and 72 current smokers) who underwent measurement of DLCOsb in the decade 1985-1994 at the Lung Function Laboratory of our institute. With respect to the unadjusted DLCOsb value, significant small differences were found for all the corrected formulas, ranging from -0.18 to 1.48 ml/min/mm Hg in men and from -0.24 to 1.57 ml/min/mm Hg in women. After computing the percent change of DLCOsb [(unadjusted-adjusted value) x 100/unadjusted value], we observed that the correction for VD caused an underestimation of DLCOsb of about 5.8% in men and 7.7% in women. However, when all the corrections were considered, these figures decreased to about 0.9% in males and 2.9% in females. Regarding specifically the correction for Hb, the adjusted value was slightly lower in men, while it was some-what higher in women, with respect to the unadjusted DLCOsb. In conclusion, the corrections suggested by ATS in the computation of DLCOsb, when considered altogether, seem to account for a limited proportion of test variability in usual clinical conditions, especially in males.