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INTRODUCTION Chronic dyspnoea that remains unexplained after resting pulmonary function and cardiovascular testing is a common problem in clinical practice. The aim of this study was to determine the utility of cardiopulmonary exercise testing (CPET) in the diagnosis of unexplained dyspnoea. METHODS This retrospective single-centre study included consecutive patients with dyspnoea who had normal resting cardiopulmonary examinations (including chest X-ray, electrocardiography, pulmonary function tests [PFTs], and cardiac ultrasound). CPET was performed using a cycle ergometer with analysis of blood gases. The results were interpreted as being most likely due to one of the six pathophysiological mechanisms shown below. Consensus required agreement between at least three of the authors. RESULTS Of the 194 patients included (median age 53 years, sex-ratio (M:F) 0.83, mean body mass index 27.3±5.36kg/m2), 32% of the test profiles were compatible with deconditioning, 20% with inappropriate hyperventilation (without gas exchange abnormalities), 18% with disorders of gas exchange, 13% with sub-maximal CPET, 9% with cardiovascular anomalies, and 8% with normal CPET. Of the patients with gas exchange abnormalities, the most common causes were bronchiectasis (6), emphysema (6), recent pneumonia (2), and diffuse interstitial pneumonitis (2). Ten of the patients with cardiovascular abnormalities had chronotropic insufficiencies, 5 had excessive tension responses, and 3 had disorders of rhythm or repolarisation. CONCLUSIONS CPET may greatly facilitate the diagnosis of unexplained dyspnoea. More than 50% of the dyspnoea cases examined here were due to deconditioning or hyperventilation syndrome and would benefit from a simple pulmonary rehabilitation program.