The incidence and prevalence of chronic thromboembolic pulmonary hypertension (CTEPH) are yet to be accurately determined and may be significantly underestimated. Historically, the occurrence of CTEPH in patients diagnosed with acute pulmonary embolism (PE) has been considered rare. Data from autopsy studies estimated the incidence of CTEPH at 1-3% overall and at 0.1-0.5% in patients surviving acute PE. Data indicate that each year in the United States, approximately 600,000 individuals suffer an acute pulmonary embolic event and that the annual number of new CTEPH cases in the United States is between 500 and 2,500. This may underestimate the true frequency of CTEPH because the disease is often misdiagnosed due to nonspecific symptoms and variable disease course. A monocenter, prospective, longitudinal study assessing symptomatic CTEPH in patients with acute PE but without prior venous thromboembolism recently estimated the cumulative incidence of CTEPH to be 1.0% 6 mo after PE, 3.1% after 1 yr, and 3.8% after 2 yr; overall post-PE incidence was approximately 3%. Further studies are needed to better define the true rate of CTEPH. Acute embolic events can occur without symptoms, and symptomatic PE is often overlooked or misdiagnosed in practice. CTEPH is often identified during diagnostic work-up in patients with unexplained pulmonary hypertension, many of whom lack medical history suggesting previous PE. In a recent study in 142 consecutive patients with CTEPH, 90 (63%) had no previous history of symptomatic venous thromboembolism. Further prospective epidemiologic studies are needed to better define the incidence and prevalence of CTEPH.