While airways reactivity is among the characteristics of asthma, it is not considered a sufficient condition diagnostically and the methacholine challenge is a non-specific diagnostic aid in cases of chronic cough and reactive airways disease. The aim of this cross-sectional study was to determine the metacholine response positivity and diagnosis of asthma in patients with chronic cough presenting to a hospital in Tehran during 2007 and 2008. Of 101 patients with chronic cough (with no history of sinusitis, recent pulmonary infection, bronchitis, gasteroesophageal reflux or underlying pulmonary conditions), 51.5% showed reactive airways disease to the methacholine test, 40.6% were unreactive and 7.9% were indeterminate. A positive methacholine challenge test was positively correlated with new wheezing. Although the methacholine challenge test is not a primary test for evaluating chronic cough, if no other reason for chronic cough is found, it may be a guiding test for asthma.