['Right ventricular failure does not explain all cases of death in patients with chronic pulmonary hypertension. Searching for alternative explanations, we evaluated the prognostic significance of main pulmonary artery (PA) dilatation in patients with pulmonary arterial hypertension (PAH) or chronic thromboembolic pulmonary hypertension (CTEPH).', 'A retrospective outcome analysis was made of 264 patients (aged 46 ± 17 years; women, 69%; PAH, 82%) who underwent both CT scan measurement of the PA and right-sided heart catheterization (mean PA pressure, 57.6 ± 16.5 mm Hg) at initial evaluation.', 'The diameter of the PA ranged from 28 to 120 mm (mean, 39 ± 8.6 mm; median, 38 mm) and was largest in patients with unrepaired congenital defects (42.6 ± 7.6 mm). Pulmonary pulse pressure (P = .04), lower age (P = .03), and duration of symptoms (P &lt; .001) were independently but weakly related to PA diameter. During follow-up (median, 38 months), 99 patients (37%) died. Of these 99 deaths, 73 (74%) were due to heart failure or comorbidities, and 26 (26%) were unexpected deaths (UE-Ds). PA diameter (hazard ratio [HR], 1.06 per 1 mm; 95% CI, 1.03-1.08), heart rate (HR, 1.30 per 10 beats/min; 95% CI, 1.01-1.66), and systolic pulmonary arterial pressure (HR, 1.02 per 1 mm Hg; 95% CI, 1.01-1.04) were the only independent predictors of UE-D and differed from the usual predictors found in the study group for all-cause mortality. PA diameter ≥ 48 mm had 95% specificity and 39% sensitivity and carried 7.5 times higher risk of UE-D (95% CI, 3.4-16.5; P &lt; .0001) during follow-up.'] Does pulmonary artery dilatation correlate with the risk of unexpected death in chronic arterial or thromboembolic pulmonary hypertension?