Treatments for pulmonary arterial hypertension have been mainly studied in patients with advanced disease (WHO functional class [FC] III and IV). This study was designed to assess the effect of the dual endothelin receptor antagonist bosentan in patients with WHO FC II pulmonary arterial hypertension. Patients with WHO FC II pulmonary arterial hypertension aged 12 years or over with 6-min walk distance of less than 80% of the normal predicted value or less than 500 m associated with a Borg dyspnoea index of 2 or greater were enrolled in this double-blind, placebo-controlled, multicentre trial. 185 patients were randomly assigned to receive bosentan (n=93) or placebo (n=92) for the 6-month double-blind treatment period via a centralised integrated voice recognition system. Primary endpoints were pulmonary vascular resistance at month 6 expressed as percentage of baseline and change from baseline to month 6 in 6-min walk distance. Analyses of the primary endpoints were done with all randomised patients who had a valid baseline assessment and an assessment or an imputed value for month 6. This trial was registered with ClinicalTrials.gov, number NCT00091715. Analyses were done with 168 patients (80 in the bosentan group, 88 in the placebo group) for pulmonary vascular resistance and with 177 (86 and 91) for 6-min walking distance. At month 6, geometric mean pulmonary vascular resistance was 83.2% (95% CI 73.8-93.7) of the baseline value in the bosentan group and 107.5% (97.6-118.4) of the baseline value in the placebo group (treatment effect -22.6%, 95% CI -33.5 to -10.0; p<0.0001). Mean 6-min walk distance increased from baseline in the bosentan group (11.2 m, 95% CI -4.6 to 27.0) and decreased in the placebo group (-7.9 m, -24.3 to 8.5), with a mean treatment effect of 19.1 m (95% CI 3.6-41.8; p=0.0758). 12 (13%) patients in the bosentan group and eight (9%) in the placebo group reported serious adverse events, the most common of which were syncope in the bosentan group and right ventricular failure in the placebo group. Bosentan treatment could be beneficial for patients with WHO FC II pulmonary arterial hypertension.