AIM: To determine immunocytochemically whether preterm and newborn infants with necrotising enterocolitis (NEC) show differences in numbers of lysozyme positive Paneth cells compared with normal controls, and to relate the findings to the possibility that lysozyme deficiency may facilitate the bacterial infections thought to be associated with this condition. METHODS: Tissues from 10 infants with NEC and from 11 matched controls were sectioned and stained immunocytochemically for lysozyme. Differences in the numbers of Paneth cells and degree of lysozyme positivity in the tissues were assessed. RESULTS: Tissues from NEC patients showed no, or very few, lysozyme positive Paneth cells, whereas controls showed strong positive staining. CONCLUSIONS: A deficiency or developmental defect in Paneth cells, resulting in an absence of lysozyme, may render the intestine more susceptible to bacterial infection, allowing organisms to adhere and translocate across the mucosa. Such enhancement of infection may contribute to the pathogenesis of NEC.