What is Known: • Clinical deterioration, despite optimal medical treatment, is a relative indication for surgery in infants with necrotizing enterocolitis. • Hyponatremia is a common condition in preterm infants from the second week of life.

What is New: • Hyponatremia and a decrease in plasma sodium level at the onset of necrotizing enterocolitis are positively associated with need of surgery or death within 2 weeks. • In infants with necrotizing enterocolitis, without pneumoperitoneum, where clinical deterioration despite optimal medical treatment is the only indication for surgery, hyponatremia, or a decrease in plasma sodium level can predict the severity of the disease.