For this article, the literature on the pathophysiology, clinical features, natural history, prognosis, and management of the Eisenmenger syndrome in adults was reviewed. English-language articles from 1966 to the present were identified through a search of the MEDLINE database by using the terms Eisenmenger, congenital heart disease, and pulmonary hypertension. Selected cross-referenced articles were also included. Articles on the pathophysiology, clinical presentation, evaluation, natural history, complications, and treatment of the Eisenmenger syndrome in adults were selected, and descriptive and analytical data relevant to the practicing physician were manually extracted. The Eisenmenger syndrome is characterized by elevated pulmonary vascular resistance and right-to-left shunting of blood through a systemic-to-pulmonary circulation connection. Most patients with the syndrome survive for 20 to 30 years. The hemostatic changes associated with the syndrome may lead to thromboembolic events, cerebrovascular complications, or the hyperviscosity syndrome. Erythrocytosis is present in most patients, but excessive phlebotomy may cause microcytosis and exacerbate the symptoms of hyperviscosity. Other complications associated with the Eisenmenger syndrome include hemoptysis, gout, cholelithiasis, hypertrophic osteoarthropathy, and decreased renal function. Pregnancy or noncardiac surgery is associated with a high mortality rate in patients with the Eisenmenger syndrome. Because most pediatric patients with the Eisenmenger syndrome survive to adulthood, primary care physicians should have a thorough understanding of the syndrome; its associated complications; and medical and surgical management, especially with regard to the appropriate timing of phlebotomy and lung or heart-lung transplantation. In addition, patients with the syndrome should undergo routine follow-up at a tertiary care center that has physicians and nurses with special expertise in congenital heart disease. In patients with the Eisenmenger syndrome who are pregnant or require noncardiac surgery, a multidisciplinary approach should be used to reduce the excessive mortality associated with these conditions.