IMPORTANCE There is concern that long-acting muscarinic antagonists increase cardiovascular morbidity or mortality in patients with chronic obstructive pulmonary disease (COPD). OBJECTIVE To determine the cardiovascular safety (noninferiority) and efficacy (superiority) of aclidinium bromide, 400 μg twice daily, in patients with COPD and cardiovascular disease or risk factors. DESIGN, SETTING, AND PARTICIPANTS Multicenter, randomized, placebo-controlled, double-blind, parallel-design study conducted at 522 sites in North America. A total of 3630 patients with moderate to very severe COPD and either a history of cardiovascular disease or at least 2 atherothrombotic risk factors were randomized; follow-up occurred for up to 3 years until at least 122 major adverse cardiovascular events (MACE) occurred. The first patient was enrolled on October 16, 2013 and the last on August 22, 2016. The final patient completed follow-up on September 21, 2017. INTERVENTIONS Patients were randomized to receive aclidinium (n = 1812) or placebo (n = 1818) by dry-powder inhaler, twice daily for up to 3 years. MAIN OUTCOMES AND MEASURES The primary safety end point was time to first MACE over up to 3 years (hazard ratio [HR] 1-sided 97.5% CI noninferiority margin = 1.8). The primary efficacy end point was the annual COPD exacerbation rate during the first year of treatment. Secondary outcomes included an expanded MACE definition (time to first MACE or serious cardiovascular event of interest) and annual rate of exacerbations requiring hospitalization. RESULTS Among 3589 patients analyzed (mean age, 67.2 years; 58.7% male), 2537 (70.7%) completed the study. Of these, 69 (3.9%) aclidinium and 76 (4.2%) placebo patients had a MACE (HR, 0.89; 1-sided 97.5% CI, 0-1.23); the expanded MACE definition included 168 (9.4%) aclidinium vs 160 (8.9%) placebo patients with events (HR, 1.03; 1-sided 97.5% CI, 0-1.28). Annual moderate to severe exacerbation rates (aclidinium, 0.44; placebo, 0.57; rate ratio, 0.78; 2-sided 95% CI, 0.68-0.89; P < .001) and rate of exacerbations requiring hospitalization (aclidinium, 0.07; placebo, 0.10; rate ratio, 0.65; 2-sided 95% CI, 0.48-0.89; P = .006) decreased significantly with aclidinium vs placebo. The most common adverse events were pneumonia (aclidinium, 109 events [6.1%]; placebo, 105 events [5.8%]), urinary tract infection (aclidinium, 93 events [5.2%]; placebo, 89 events [5.0%]), and upper respiratory tract infection (aclidinium, 86 events [4.8%]; placebo, 101 events [5.6%]). CONCLUSIONS AND RELEVANCE Among patients with COPD and increased cardiovascular risk, aclidinium was noninferior to placebo for risk of MACE over 3 years. The rate of moderate to severe COPD exacerbations was reduced over the first year.