OBJECTIVE Atrial natriuretic peptide (ANP) is a cardiac hormone with many biological effects. Hypersecretion may lead to hyponatremia. This study examined the umbilical ANP levels in high risk neonates. METHODS A total of 117 high risk neonates born between June, 2004 and June, 2005 were divided into Simple asphyxia and Normal score groups according to their Apgar's scores. The Simple asphyxia group was subdivided into Mild (n=20) and Severe asphyxia groups (n=17), and the Normal score group was subdivided into Infection (n=25) and Non-infection groups (n=55). Forty normal neonates were used as the Control group. The samples of umbilical cord blood were collected at delivery and the umbilical ANP levels were measured by radioimmunoassay. Meanwhile the sodium levels in the peripheral vein were measured. RESULTS The mean umbilical ANP levels in high risk neonates were significantly higher than those in the normal neonates. A more significant increase of the umbilical ANP level was observed in premature infants (1.46 +/- 0.39 ng/mL), and neonates with serious infection (1.16 +/- 0.35 ng/mL) and with severe asphyxia (2.12 +/- 0.46 ng/mL) compared with the normal neonates (0.62 +/- 0.33 ng/mL; P < 0.01). The serum sodium level was negatively correlated with the umbilical ANP level (r=-0.99, P < 0.01). CONCLUSIONS The umbilical ANP levels increased significantly in the high risk neonates, suggesting high risk neonates are susceptible to hyponatremia.