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Background: Airway resistance (R-AW) and specific airway conductance (sG(AW)) are measures that reflect the patency of airways. Little is known of the variability of these measures between different lung diseases. This study investigated the contribution of R-AW and sG(AW) to a diagnosis of obstructive airways disease and their role in differentiating asthma from COPD. Methods: 976 subjects admitted for the first time to a pulmonary practice in Belgium were included. Clinical diagnoses were based on complete pulmonary function tests and supported by investigations of physicians' discretion. 651 subjects had a final diagnosis of obstructive diseases, 168 had another respiratory disease and 157 subjects had no respiratory disease (healthy controls). Results: R-AW and sG(AW) were significantly different (p < 0.0001) between obstructive and other groups. Abnormal R-AW and sG(AW) were found in 39 % and 18 % of the population, respectively, in which 81 % and 90 % had diagnosed airway obstruction. Multiple regression revealed sG(AW) to be a significant and independent predictor of an obstructive disorder. To differentiate asthma from COPD, R-AW was found to be more relevant and statistically significant. In asthma patients with normal FEV1/FVC ratio, both R-AW and sG(AW) were more specific than sensitive diagnostic tests in differentiating asthma from healthy subjects. Conclusions: R-AW and sG(AW) are significant factors that contribute to the diagnosis and differentiation of obstructive airways diseases.