Chronic Fatigue Syndrome (CFS), also known as myalgic encephalomyelitis (ME), is a complicated disease characterized by extreme fatigue of at least six months duration. It is necessary to rule out the organic cause of CFS in the paediatric patient in order not to overlook the preventable or the treatable condition. In our case report we present a case of a patient with a surprising origin diagnosed by cardiopulmonary exercise testing.