Dyspnea and exercise limitation are ubiquitous symptoms in heart failure; the role of pulmonary factors has not been defined. We studied patients with severe left ventricular dysfunction at upright rest and during cycle ergometry. Arterial oxygen tension and saturation were normal at rest and exercise despite mild abnormalities in Qs/Qt and P(A-a)O2, as well as the effects of low S-vO2. Ventilation was increased at rest and exercise because of increased VD/VT and alveolar hyperventilation. The basis of these abnormalities is unknown. We concluded that when considering the symptoms of dyspnea and exercise limitation, arterial oxygenation is probably not important, but excessive ventilation may be a contributing factor.