Breathlessness arises from increased medullary respiratory center activity projecting to the forebrain (respiratory corollary discharge hypothesis). Subjects with congenital central hypoventilation syndrome (CCHS) lack the normal hyperpnea and breathlessness during hypercapnia. The corollary discharge hypothesis predicts that if CCHS subjects have normal hyperpnea during exercise, they will experience normal breathlessness during exercise. To test this, we studied four CCHS subjects and six matched controls during an exhausting constant-load cycling test requiring substantial anaerobiosis. CCHS subjects rated significantly less breathlessness at the end of the test than controls, but ventilation (index of respiratory corollary discharge) was also somewhat lower in CCHS (not significant). In both groups, breathlessness increased disproportionately more than ventilation towards the end of exercise. These data failed to disprove the corollary discharge hypothesis of breathlessness, but do suggest that the relationship between ventilation and breathlessness is non-linear and/or that projections of chemoreceptor afferents to the forebrain (presumed lacking in CCHS) is one source of breathlessness in normals.