Rationale: Epidemiological studies in older individuals have found an association between the use of angiotensin-converting enzyme (ACE) inhibition (ACE-I) therapy and preserved locomotor muscle mass, strength, and walking speed. ACE-I therapy might therefore have a role in the context of pulmonary rehabilitation (PR).Objectives: To investigate the hypothesis that enalapril, an ACE inhibitor, would augment the improvement in exercise capacity seen during PR.Methods: We performed a double-blind, placebo-controlled, parallel-group randomized controlled trial. Patients with chronic obstructive pulmonary disease, who had at least moderate airflow obstruction and were taking part in PR, were randomized to either 10 weeks of therapy with an ACE inhibitor (10 mg enalapril) or placebo.Measurements and Main Results: The primary outcome measurement was the change in peak power (assessed using cycle ergometry) from baseline. Eighty patients were enrolled, 78 were randomized (age 67 6 8 years; FEV 1 48 6 21% predicted), and 65 completed the trial (34 on placebo, 31 on the ACE inhibitor). The ACE inhibitor-treated group demonstrated a significant reduction in systolic blood pressure (D, 216 mm Hg; 95% confidence interval [CI], 222 to 211) and serum ACE activity (D, 218 IU/L; 95% CI, 223 to 212) versus placebo (between-group differences, P , 0.0001). Peak power increased significantly more in the placebo group (placebo D, 19 W; 95% CI, 5 to 13 vs. ACE-I D, 11 W; 95% CI, 22 to 4; between-group difference, 8 W; 95% CI, 3 to 13; P = 0.001). There was no significant betweengroup difference in quadriceps strength or health-related quality of life.Conclusions: Use of the ACE inhibitor enalapril, together with a program of PR, in patients without an established indication for ACE-I, reduced the peak work rate response to exercise training in patients with chronic obstructive pulmonary disease.