A review of necropsy material reveals that idiopathic necrotizing enterocolitis was present in 13 of 427 (3.04%) premature infants. In seven of the 13 infants, the diagnosis had been clinically made or strongly suspected. All 13 patients showed variable degrees of pulmonary atelectasis with or without hyaline membranes. The association of necrotizing enterocolitis with the respiratory distress syndrome plus the high incidence of widespread hemorrhagic phenomena observed in these patients suggests that perinatal hypoxia and its effects on vessels may be an important pathogenetic factor. Notably, the hemorrhagic manifestations were most severe in the patients with necrosis of the bowel wall and perforation. There is a possible pathogenetic role of a diving reflex-like phenomenon.