Chronic heart failure (CHF) is a common condition with a poor prognosis. It is associated with debilitating limiting symptoms even with optimal modern medical management. Foremost among these symptoms is severe exercise intolerance with marked fatigue and dyspnea at low exercise work loads. The severity of symptomatic exercise limitation varies between patients, and this appears to bear little relationship with the extent of the left ventricular systolic dysfunction measured at rest, or to markers of hemodynamic disturbance. In studying factors limiting exercise capacity in heart failure patients, it is necessary to compare their situation to factors limiting exercise in normal subjects.