In primary pulmonary hypertension PGI 2 , when added to conventional therapy, enhances early survival at 3 months and improves exercise tolerance (Long et ai, Am Rev Respir Dis 1993; 149: A538). High dose Ca ++ channel blockers may improve survival in mild to moderate disease (Rich et ai, N Eng J Med 1992; 327: 76–81). We have compared the effect of continuous infusion of PGI 2 on survival of 114 patients with pulmonary hypertetion where severity of disease was determined by a mixed venous oxygen saturation (SvO 2 ). Patients were categorized into groupl (SvO 2 ≥ 60%) and group2 (Sv0 2 < 60%) at diagnostic right heart catheterization. Majority of patients had primary pulmonary hypertension In = 90) while others had thromboembolic disease (n = 24). 67 patient were treated with PGI2, while 47 patients were treated with conventional therapy (CT) ie. anticoagulants with or without oral vasodilators. End point of the study was either death or heart-lung transplantation. Group 1(n = 58): SvO 2 ≥ 60% Group 2 (n = 56): SvO 2 < 60% Age (Y) 39.1 ± 11.8 35.1 ± 12.7 Sex F/M 39/19 31/25 PAPm(mmHg) 59.9 ± 15.6 677 ± 15.5 * RAPm(mmHg) 8.5 ± 63 13.1 ± 5.8 * Sv0 2 (%) 67.4 ± 6.1 51.2 ± 7.2 * CI (Umin/m 2 ) 2.05 ± 0.67 1.79 ± 0.46 * Mean ± SD * p < 0.05 Sv0 2 ≥ 60%(n = 58) Sv02 < 60%(n = 58) CT (n = 30) PGI 2 (n = 28) CT (n = 17) PGI 2 (n = 39) Median 1277 1061 253 666 * survival in days (390->2100) (450-1590) (30-570) (240-1470) Mean (Inter-quartile rangel) * P < 0.05, Likelihood ratio test Continuous PGl 2 infusion, in comparison to CT, improved median survival in patients with severe pulmonary hypertension who had Sv0 2 of less than 60%. However in moderate disease with Sv0 2 of ≥ 60%, PGl 2 had no advantage over CT. Continuous long term PGl 2 may be a useful treatment in selected patients with pulmonary hypertension where Sv0 2 is less than 60 percent.