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In patients with pulmonary hypertension (PH), elevated endothelin-1 levels are associated with prolonged duration of right ventricular (RV) contraction, which induces leftward ventricular septal bowing with impaired left diastolic filling. We hypothesized that baseline RV contraction duration predicts efficacy of endothelin receptor antagonist, bosentan.Eighteen PH patients (age 57, range 35–79 years, 33% male) received bosentan. Six minute walk distance (6-MWD) and echocardiography were performed at baseline and after 1 year follow-up. After 1 year of treatment, 6-MWD increased (mean 60 ± 41 m) in 67% of patients (responders). Baseline RV contraction duration was longer in responders, compared with non-responders (612 ± 66 vs. 514 ± 23 ms; P < 0.01). A baseline RV contraction duration >550 ms was associated with improved 6-MWD (sensitivity 83%, specificity 83%; P < 0.01).An improvement of 6-MWD during bosentan treatment was associated with a decrease in RV contraction duration and could be predicted by a baseline RV contraction duration >550 ms.