BACKGROUND: COPD increases the risk of cardiovascular problems. Dyspnea on exertion can be associated with COPD or heart failure or both. N-terminal-pro-brain natriuretic peptide (NT-pro-BNP) is a marker of cardiac dysfunction, and exercise testing can identify subtle heart abnormalities. OBJECTIVE: To determine whether cardiac dysfunction adds to the mechanism of dyspnea caused primarily by impaired lung function in patients with mild to moderate COPD. METHODS: With 19 COPD patients and 10 healthy control subjects we measured physiologic variables and collected venous blood samples before and during incremental and constant-work-rate exercise, and measured NT-pro-BNP. RESULTS: Peak oxygen uptake and constant-work exercise time were significantly lower in the COPD group than in the control group (16 +/- 4 mL/min/kg vs 19 +/- 6 mL/min/kg, P = .04, and 7.8 +/- 6.5 min vs 14.8 +/- 7.3 min, P = .02). Between the groups there were no significant differences in anaerobic threshold, oxygen pulse (oxygen uptake divided by heart rate), or heart-rate reserve (difference between predicted and measured maximum heart rate). Both at rest and during constant-work exercise, NT-pro-BNP was not significantly higher in the COPD group than in the control group. In the COPD patients there was no significant correlation between constant-work exercise time and NT-pro-BNP at rest or during exercise. CONCLUSIONS: Heart failure did not contribute to exercise intolerance in patients with mild to moderate COPD.