Aim. To study the clinical and hemodynamic characteristics of a group of patients with Functional Class (FC) IV pulmonary arterial hypertension (PAH) developing in the presence of diffuse connective tissue diseases (DCTD) and to evaluate the efficacy of intravenous iloprost. Subjects and methods. The study enrolled 59 patients with PAN-DCTD, including 7 who had FC IV and 8 who developed this condition during a follow-up. The diagnosis of PAH was based on pulmonary artery catheterization findings. FC IV was diagnosed using the conventional New York Heart Association classification. All the patients received PAH-specific therapy (bosentan, sildenafil); the patients with FC IV had combined therapy; 4 patients were treated with intravenous iloprost calculated with reference to 0.5—2.5 ng/kg/min for 15 days. In addition to the patients with FC IV, 3 patients with unstable FC III were given iloprost. Besides targeted therapy, all the patients received standard treatment, including diuretics, and ionotropic therapy. Results. Evaluation of hemodynamics in patients with different FCs revealed the most important differences in right atrial pressure, cardiac output, cardiac index, and pulmonary vascular resistance. A linear relationship was seen between the level of this indicator and FC, the closest correlation being for hemodynamic parameters characterizing right ventricular systolic function. There were no changes in mean pulmonary artery pressure; only the patients with FC IV were found to have its slight elevation (from 52±15 to 55±11 mm Hg). Pulmonary artery wedge pressure remained unchanged regardless of FC. Intravenous iloprost was noted to have an obvious positive effect on both clinical and hemodynamic parameters. Catheterization verified improvement in 6 out of the 7 patients; no hemodynamic changes were found in 1 patient during 15-day therapy. Conclusion. The patients with FC IV PAH-DCTD have clinical and hemodynamic features responsible for a fatal prognosis. The results of using intravenous iloprost in patients with decompensated PAH associated with scleroderma systematica convince to use its PAH-specific tablets in cases of verified clinical deterioration when taking its dosage form.