In his case report, a 74-year-old physically fit man was evaluated repeatedly for several years in the cardiology department due to dyspnoea on exertion (DOE). Several standard cardiac and pulmonary tests were performed but did not provide sufficient cause for the DOE. Lastly, the patient was evaluated with a cardiopulmonary exercise test (CPET) with simultaneous in- and expiratory gas sampling. The test revealed a low aerobic capacity due to chronotropic incompetence (CI), thus explaining the DOE. Subsequently, the patient was treated with a rate-responsive pacemaker. CPET-is an ideal test for diagnosing CI.