Sometimes a clinician tell you that a patient has severe pulmonary arterial hypertension when the plain film shows very little evidence of pulmonary arterial dilatation, as in these two cases. Resist the temptation to say that you doubt very much if the pulmonary arterial pressure is as high as the clinician thinks. The left image is of an 18 year old male with an Eisenmenger VSD. He is one of a number where the pulmonary arterial resistance has never fallen beyond neonatal values, there has never been much of a shunt, and the pulmonary arterial walls have retained their foetal thickness. As a result the central pulmonary arteries have been able to withstand the years of pulmonary hypertension and are minimally dilated. Many in this group have Downs Syndrome, but this case did not. The case on the right is of a middle aged lady with chronic, operated mitral regurgitation. The ascending aorta is prominent, and as usual it is not possible to say if it is dilated or unfolded. Actually it is unfolded, and as the ascending aorta migrates to the right it takes the horizontal part of the main pulmonary artery with it. As a result, PA dilatation might still be concealed within a reasonably normal looking upper left cardiac outline. (This is very similar to the fact that the ascending aorta can be quite widened yet the mediastinum is minimally widened - especially in Marfan's Syndrome.) Modern echocardiography is so good at evaluating PA pressure that you should challenge it infrequently.