Background Exertional dyspnea is a common symptom in patients with heart failure, and the mechanisms responsible for the symptom are unknown. The purpose of this study was to identify factors responsible for the symptom of exertional dyspnea in patients with heart failure. Methods Resting pulmonary-function tests and maximal cardiopulmonary exercise tests were performed in 71 patients with New York Heart Association functional class II-IV symptoms (mean ejection fraction 30.6%; mean age, 68 years). Results The severity of dyspnea at peak exercise, which patients rated as 3 to 10 on a 1 to 10 severity scale, did not correlate with rest or exercise hemodynamic, spirometric, or metabolic variables, including peak oxygen uptake (VO₂), minute ventilation (Ve), and respiratory rate, or with derived variables including Ve/VO₂, Ve/VCO₂, and the dyspnea index (Ve/maximum voluntary ventilation). Additionally, these variables did not differ between patients who reported limitation of exercise by dyspnea and those who were limited by fatigue. Conclusions The symptom of exertional dyspnea in patients with heart failure is not determined by abnormalities in ventilatory function or demand. (Am Heart J 1998;135:398-405.)