Aim. To study the effect of bosentan on the clinical status and cellular immunity of patients with idiopathic pulmonary hypertension (IPH). Subjects and methods. The study enrolled 35 patients with Functional Class (FC) II and IV IPH who had received conventional therapy for 3 months. All the patients took bosentan 125 mg/day for 4 weeks. Then they were randomized to have bosentan 125 or 250 mg/day (Groups 1 and 2). FC was assessed and 6-minute walk test (6MWT), transthoracic echocardiography (EchoCG), and right heart catheterization (RHC) were performed at baseline and after 12 weeks. Peripheral blood lymphocytes were immunophenotyped and the functional activity of neutrophils was determined by their ability to absorb latex particles at baseline and following 3 and 12 weeks. Results. After 12 weeks of treatment, both groups showed a statistically significant increase in the distance covered during the 6MWT. The Borg index and FC also decreased statistically significantly. EchoCG demonstrated a statistically significant reduction in systolic pulmonary artery pressure (SPAP) in Group 1; and in Group 2, SPAP was lower at baseline and remained substantially unchanged (it statistically significantly decreased as evidenced by RHC). Bosentan statistically significantly diminished pulmonary vascular resistance in both groups. No adverse reactions were noted. At baseline, the patients showed a statistically significant elevation in NKT-like lymphocytes up to 10.79±6.2%. The other indicators of peripheral blood lymphocyte phenotyping did not exceed the normal range. The count of NKT-like lymphocytes remained significantly above the normal levels throughout the follow-up in both groups and slightly dropped in Group 1 patients only after 12 weeks of treatment. In Group 2, the level of CD3+CD25+ lymphocytes rose as compared to the normal levels at 3 and 12 weeks of treatment. At 3 weeks of treatment, there was a statistically significant difference between the groups in phagocytic number. Conclusion. Bosentan treatment statistically significantly improves the clinical status of patients with IPH and it is characterized by good tolerability. Bosentan does not exceed the normal count of peripheral blood NKT-like lymphocytes, but it has a direct or indirect positive effect on the B-lymphocytic component of the immune system in patients with IPH.