['Prostacyclin (PGI(2)) and thromboxane A(2) (TxA(2)) may take part in lung pathology; high concentrations of PGI(2) may protect newborn rabbits against hyperoxic lung injury, and TxA(2) may participate in the development of bronchopulmonary dysplasia (BPD).', 'To examine in small preterm infants, the relationship between pulmonary PGI(2) and TxA(2) and respiratory distress during the early postnatal period.', 'The stable metabolite of prostacyclin, 6-keto-prostaglandinF(1 alpha), and that of thromboxane A(2), thromboxane B(2), were quantified by radioimmunoassays in 284 samples of tracheal aspirates from 48 infants (GA: 27.4+/-2.1 week, BW 959+/-334 g) during the first 12 postnatal days.', 'Mean concentration of 6-keto-prostaglandinF(1 alpha) was 414+/-31 pg/ml (mean+/-S.E.M.), and of thromboxane B(2) was 418+/-37 pg/ml. Correlations existed between 6-keto-prostaglandinF(1 alpha) and gestational age, birth weight, and the initial arterial-alveolar oxygenation ratio. Negative correlations existed between 6-keto-prostaglandinF(1 alpha) and both mean inspiratory oxygen and duration of mechanical ventilation. Indomethacin treatment was associated with lower pulmonary 6-keto-prostaglandinF(1 alpha), but not with lower TxB(2). Thromboxane B(2) correlated positively with gestational age, birth weight, and initial arterial-alveolar oxygenation ratio, and inversely with duration of mechanical ventilation.'] Is pulmonary prostacyclin associated with less severe respiratory distress in preterm infants?