The aim of this study was to determine whether the ratio of the diameters of the main pulmonary artery and of the ascending aorta (rPA), as assessed on computed tomography (CT), is predictive of pulmonary arterial hypertension (PAH). We undertook a retrospective review of 50 patients with a wide range of pulmonary and cardiovascular diseases, who had undergone both chest CT and pulmonary arterial pressure measurements at right heart catheterization. Two independent observers made measurements of the diameter of the main pulmonary artery and of the ascending aorta on a single defined CT section. Body surface area (BSA, n = 48), pulmonary arteriolar resistance (n = 39), total lung capacity (n = 40), and aortic pressures (n = 50) were also recorded. rPA and pulmonary arterial diameter (dPA) were positively related to mean pulmonary artery pressure (Rs = 0.74, p < 0.0005 for both analyses). For patients younger than 50 years of age, mean pulmonary artery pressure correlated more strongly with rPA than dPA (Rs = 0.77, p < 0.00005, compared with Rs = 0.59, p < 0.005); and vice versa for patients older than 50 years of age (Rs = 0.63, p < 0.005, compared with Rs = 0.75, p < 0.00005). Using a mean pulmonary artery pressure greater than 20 mm Hg as indicative of PAH and a value of rPA > 1, the sensitivity, specificity, and positive and negative predictive values for determining PAH were 70% (26/37), 92% (12/13), 96% (26/27), and 52% (12/23), respectively. On multivariate analysis, rPA was positively related to mean pulmonary artery pressure (p < 0.0005), and negatively related to age (p < 0.0005), but was not related to BSA. By contrast, dPA showed some dependency on BSA (p < 0.0005), as well as on mean pulmonary arterial pressure. In patients younger than 50 years of age, we have found a strong correlation between rPA and mean pulmonary artery pressure in a heterogeneous study population, and this relationship is independent of BSA and sex. The presence of the sign "rPA > 1" is simple in practical CT reading to determine; if this is identified, there is a very high probability of pulmonary arterial hypertension, and clinicians should be alerted to this possibility.