Pulmonary hypertension is a rare disease with a poor prognosis. It was first described in the late 19th century as a clinical-pathological syndrome characterised by obstruction of the small pulmonary arteries and right ventricular hypertrophy in patients presenting with severe dyspnoea and cyanosis. After the development of right heart catheterisation in the second half of the 20th century, it was found that many diseases could cause pulmonary hypertension, which is now recognised to be high blood pressure in the arteries that supply the lungs. In the 1960s, an epidemic of pulmonary hypertension caused by appetite suppressants initiated a systematic collection of information on pulmonary hypertension, leading to the first international classification of pulmonary hypertension. Increased understanding of the pathogenesis of the various forms of pulmonary hypertension has led to novel treatments and holds promise for the future.