Cord serum sodium levels were estimated in 96 full term singletons delivered by spontaneous vertex delivery. Thirty-two of these infants had cord serum sodium of less than 130 mmol/L and 64 had cord serum sodium of > or = 130 mmol/L. Serum bilirubin estimated on day 4 of life of the babies demonstrated mean unconjugated bilirubin levels of 105.7 mmol/L (S.D.:87.04) and 89.4 mmol/L (S.D.:66.18) in the hyponatraemic and normonatraemic groups respectively (P < 0.05). The study also demonstrated a higher cord sodium and lower serum unconjugated bilirubin in those babies whose mothers did not receive parenteral fluids. It is suggested that hyponatraemia should be excluded in the aetiology of neonatal jaundice.