Patent ductus arteriosus (PDA) is an important cause of morbidity in extremely preterm infants. As increased plasma brain natriuretic peptide (BNP) is a common feature of adult cardiac disease, we investigated the value of plasma BNP concentration as a predictor of haemodynamically significant PDA in very preterm infants. We studied 18 preterm infants (12 male) of median gestational age 30 weeks (range 24-34), median birth weight 1.46 kg (0.54-2.13) and 11 healthy term controls. Plasma BNP levels were measured by double-antibody radioimmunoassay on days 3, 5 and 7 of life, and an echocardiogram was performed on day 7. Six infants of median gestation 26 weeks (26-30), median birth weight 0.92 kg (0.54-1.04) had PDA proven by echocardiography on day 7. BNP concentrations (pg/ml) on day 3 were significantly higher in these infants than in the remaining twelve [median 2012 (786-2759) versus 42 (7-704), P<0.001]. In four infants PDA was treated successfully (one surgically, three with non-steroidal anti-inflammatory drugs). Two had haemodynamically insignificant closing ducts. In these infants with therapeutic or spontaneous resolution of a PDA, plasma BNP fell to normal values [median after treatment 9 pg/ml (8-27)]. Early measurement of plasma BNP in the first few days of life is a useful method for predicting those preterm infants who may require intervention for PDA.