Pulmonary diffusing capacity (lung transfer factor for carbon monoxide) has been assessed by the single breath (TLCOSB) and steady state (TLCOSS) techniques in well matched 90 females, 31 non-smoker and 29 smoker males, 18-50 years of age. Both TLCOSB and TLCOSS are significantly lower in females compared to non-smoker males (P < 0.001). Tobacco smoking statistically significantly reduces TLCOSB as well as TLCOSS in smokers as compared to non-smokers. There is a statistically significant correlation of age with TLCOSS and TLCOSB in all the three groups (r = -0.702, -0.360 and 0.300 for TLCOSB and r = -0.481, -0.355 and 0.380 for TLCOSS in non-smoker males, smoker males and females respectively). TLCOSB is 30.43 +/- 4.89, 27.29 +/- 4.54 and 26.13 +/- 3.60 ml/mmHg/min, while TLCOSS is 19.47 +/- 5.26, 16.69 +/- 3.27 and 18.24 +/- 3.78 ml/mm/Hg/min in non-smoker males, smoker males and females respectively. A fairly good correlation between the TLCOSB and TLCOSS in male, both non-smoker and smoker, as well as the female subjects was observed. TLCOSS is lower than TLCOSB in all the three groups. Even in smokers of moderate intensity both of these tests are influenced to a nearly similar extent.