Background - Chronic obstructive pulmonary disease causes significant morbidity and concomitant pulmonary vascular disease and cardiac dysfunction are associated with poor prognosis. Computed tomography-detected relative pulmonary artery (PA) enlargement defined as a PA to ascending aorta diameter ratio >1 (PA:A>1) is a marker for pulmonary hypertension and predicts chronic obstructive pulmonary disease exacerbations. However, little is known about the relationship between the PA:A ratio, pulmonary blood volume, and cardiac function. Methods and Results - A single-center prospective cohort study of patients with chronic obstructive pulmonary disease was conducted. Clinical characteristics and computed tomography metrics, including the PA:A and pulmonary blood vessel volume, were measured. Ventricular functions, volumes, and dimensions were measured by cine cardiac MRI with 3-dimensional analysis. Linear regression examined the relationships between clinical characteristics, computed tomography and cardiac MRI metrics, and 6-minute walk distance. Twenty-four patients were evaluated and those with PA:A>1 had higher right ventricular (RV) end-diastolic and end-systolic volume indices accompanied by lower RV ejection fraction (52±7% versus 60±9%; P=0.04). The PA:A correlated inversely with total intraparenchymal pulmonary blood vessel volume and the volume of distal vessels with a cross-sectional area of <5 mm2. Lower forced expiratory volume, PA:A>1, and hyperinflation correlated with reduced RV ejection fraction. Both PA diameter and reduced RV ejection fraction were independently associated with reduced 6-minute walk distance. Conclusions - The loss of blood volume in distal pulmonary vessels is associated with PA enlargement on computed tomography. Cardiac MRI detects early RV dysfunction and remodeling in nonsevere chronic obstructive pulmonary disease patients with a PA:A>1. Both RV dysfunction and PA enlargement are independently associated with reduced walk distance.