Chronic obstructive pulmonary disease (COPD) is a complex condition defined by progressive airflow limitation in response to noxious stimuli, inflammation, and vascular changes. COPD exacerbations are critical events in the natural history of the disease, accounting for the majority of disease burden, cost, and mortality. Pulmonary vascular disease is an important risk factor for disease progression and exacerbation risk. Relative pulmonary artery enlargement on computed tomography scan, defined by a pulmonary artery to aortic (PA:A) ratio > 1, has been evaluated as a marker of pulmonary vascular disease. The PA:A ratio can be measured reliably independent of electrocardiographic gating or the use of contrast, and in healthy patients a PA:A ratio >0.9 is considered to be abnormal. The PA:A ratio has been compared with invasive hemodynamic parameters, primarily mean pulmonary artery pressure in various disease conditions and is more strongly correlated with mean pulmonary artery pressure in obstructive as compared with interstitial lung disease. In patients without known cardiac or pulmonary disease, the PA:A ratio is predictive of mortality, while in COPD, an elevated PA:A ratio is correlated with increased exacerbation risk, outperforming other well established predictors of these events. Future studies should be aimed at determining the stability of the metric over time and evaluating the utility of the PA:A ratio in guiding specific therapies. © 2013 Wells and Dransfield.