['Primary pulmonary hypertension (PPH) is a rare disease of unknown etiology that is characterized by a poor prognosis. This study was undertaken to investigate possible correlations between endothelin (ET)-1 and big ET-1 plasma levels and the severity of PPH.', 'Sixteen consecutive patients with PPH were included.', 'Hemodynamics of patients with PPH were measured by right-heart catheterization, and a 6-min walk test was performed.', 'Plasma levels of the biologically active peptide ET-1 and its precursor big ET-1 were determined in blood samples from the pulmonary artery, peripheral artery, and peripheral vein by radioimmunoassay.', 'A strong correlation was shown between pulmonary vascular resistance, mean pulmonary artery pressure, cardiac output, cardiac index, 6-min walk data, and elevated plasma levels of big ET-1 as well as mature ET-1 plasma levels at all sites of blood sampling (p < 0.01 and p < 0.05, respectively).'] Are big endothelin-1 and endothelin-1 plasma levels correlated with the severity of primary pulmonary hypertension?