Circulating plasma platelet activating factor in persistent pulmonary hypertension of the newborn. Platelet activating factor (PAF) is an endogenous phospholipid mediator that causes pulmonary hypertension and thrombocytopenia in experimental animal models. To investigate circulating PAF in persistent pulmonary hypertension of the newborn (PPHN), we studied PAF and its degradative enzyme, acetylhydrolase. Thirteen neonates with PPHN, diagnosed by routine clinical methods including echocardiography, were compared to six age-matched control patients with respiratory distress. Overall, plasma PAF levels were elevated in patients with PPHN compared to control patients (20.1 +/- 3.9 versus 1.6 +/- 0.7 ng/ml, p less than 0.01). In addition, plasma PAF concentrations in patients with PPHN correlated with the severity of disease as defined by the delta AaPO2 (r = 0.65, p = 0.015). In three patients with elevated PAF levels, as the clinical status improved, the plasma PAF values decreased. Acetylhydrolase activity was similar in both groups (3.96 +/- 0.90 versus 3.78 +/- 1.44 nmol/ml/min, p = NS). We conclude that PAF production is increased in PPHN and that abnormal production of PAF may be associated with pulmonary hypertension. 