BACKGROUND: Pulmonary artery (PA) dilatation is usually observed in patients with pulmonary hypertension (PH), but a PA aneurysm (PA diameter > 40mm) is rare. The difference between characteristics of patients with and those without progression of PA diameter remains poorly understood. We assessed the changes in PA diameter in patients with PH coexisting with and without a PA aneurysm. METHODS: We investigated the changes in PA diameter by multi-detector computed tomography performed twice with an interval of more than one year in 44 patients with PH. Seventeen patients had a PA aneurysm and 27 patients did not have a PA aneurysm at baseline. RESULTS: The median follow-up period was 3.6 years. All patients received medical or invasive treatment for PH. At baseline, main PA diameters were 52±15mm in patients with a PA aneurysm and 33±3mm in patients without a PA aneurysm. Mean PA pressure was higher in patients with a PA aneurysm than in those without a PA aneurysm (61±15mmHg vs. 51±16mmHg, p=0.04). At follow-up, mean PA pressure significantly decreased in both patients with a PA aneurysm (44±11mmHg) and patients without a PA aneurysm (41±18mmHg). Main PA diameter significantly increased in patients with a PA aneurysm (65±28mm, change ratio: 23.3%), while it did not increase in patients without a PA aneurysm (32±3mm, change ratio: -3.1%). CONCLUSIONS: PA dilatation progressed in patients with a PA aneurysm despite treatment of PH. The progression of PA dilatation is independent of reduction of PA pressure by PH treatment.