Background: Enlargement of the hilar branches of the pulmonary artery, in particular of the right descending pulmonary artery, has been accepted as a sign of pulmonary hypertension (PH). To clarify its diagnostic value, we investigated the relationship between right descending pulmonary artery diameter (RDPAD) and systolic pulmonary artery pressure (SPAP). Methods: In a total of 229 subjects enrolled, SPAP was estimated by transthoracic Doppler echocardiography. PH was defined as SPAP greater than 30 mmHg. RDPAD was measured on a chest X-ray (postero-anterior view) taken within one month of the echocardiography. We applied two-sample Student's t-test to test the difference of RDPAD between groups with and without PH. Linear regression analysis was performed to assess the relationship between SPAP and RDPAD. Receiver operating characteristic (ROC) curve was done to seek a potential cut-off point of RDPAD that discriminated between PH and non-PH. Results: Two-sample Student's t-test revealed RDPAD were higher in the PH group than the non-PH group (p=0.024). Linear regression analysis showed SPAP=20.9 + 0.53(superscript *)RDPAD (p=0.047), which implied a positive association between RDPAD and SPAP. However, ROC curve analysis did not find any satisfactory cut-off point of RDPAD for predicting PH. Even with RDPAD value of mean + 2SD of the non-PH group, its sensitivity and specificity, positive predictive value, negative predictive value, overall accuracy, positive likelihood ratio, and negative likelihood value were 6.7%, 97.9%, 66.8%, 62.3%, 62.5%, 3.1, and 0.95, respectively. Conclusion: Our study demonstrated RDPAD on the chest radiographic film significantly correlated with echocardiography-estimated SPAE But as a diagnostic tool, RDPAD was not good enough to predict PH.