We sought to determine whether patients with precapillary pulmonary hypertension show elevated serum levels of prolactin (PRL) and its 16-kDa N-terminal fragment (16-kDa PRL) and whether there is any correlation to measures of prognosis.Twenty-eight patients with idiopathic pulmonary artery hypertension, 15 with peripheral chronic thromboembolic pulmonary hypertension, and 4 with portopulmonary hypertension Child-Pugh class A were included. Our control subjects were 56 blood donors. Total prolactin was measured with an immunoluminometric assay. Antibodies against epitope C detected only the intact prolactin before it was split. The 16-kDa PRL was calculated from the difference between total and intact prolactin.Prolactin was significantly (P=0.009) higher in the study group (median, 190 mU/L; interquartile range, 162 mU/L) than in the control group (median, 140 mU/L; interquartile range, 91 mU/L). The 16-kDa PRL was significantly elevated in the study group (P=0.046). Prolactin and 16-kDa PRL correlated inversely with the 6-minute-walk distance (P <0.01) and with peak oxygen uptake during exercise (P <0.005).Serum levels of prolactin and 16-kDa PRL were significantly higher in patients with precapillary pulmonary hypertension and were inversely correlated with 6-minute-walk distance and peak oxygen uptake.These results indicate that prolactin and 16-kDa PRL might play a role in the pathophysiology of precapillary pulmonary hypertension.