Epoprostenol has markedly improved the treatment of pulmonary arterial hypertension, although predictors of outcome with epoprostenol are not well characterized. From June 1995 through August 2001, 91 patients with pulmonary arterial hypertension were treated with epoprostenol at our institution. We analyzed the effects of long-term epoprostenol treatment to determine features associated with outcome. Predictors of worse outcome included older age of disease onset (hazard ratio 3.2, 95% confidence interval 1.32-7.76 for patients above the median age of 44 years), World Health Organization functional Class IV, either at baseline or follow-up, (3.07, 1.42-6.62 compared with functional Class I, II, and III), and scleroderma spectrum of disease (2.32, 1.08-4.99). There were no baseline or follow-up hemodynamic factors predictive of outcome. Our results indicate that treatment with epoprostenol improves survival in patients with Primary Pulmonary Hypertension compared with that predicted by the National Institutes of Health Primary Pulmonary Hypertension Registry's survival equation and that their survival is significantly better than that of patients with scleroderma spectrum of disease (p = 0.001). Older patients treated with epoprostenol have significantly shorter survival, regardless of etiology.