Leisure-time and elite athletes often seek sports medical advice for inadequate exertional dyspnea and loss of performance. The work-up has to rule-out underlying cardiac pathologies that are associated with sudden cardiac death, although commonly the symptoms are training- and not disease-related. Cardiopulmonary exercise testing (CPET) helps to differentiate between cardiac and pulmonary causes and guides further diagnostic and therapy. This article illustrates the potential of CPET in three clinical cases.