Perforation of the appendix in the neonatal period may be a complication of neonatal necrotizing enterocolitis and should be differentiated from perforating appendicitis in later life. A patient is presented together with a review of the literature to illustrate this concept. Perforation of the appendix occurred in a 12-day-old preterm baby. Th cause of this perforation is assumed to be localized full thickness necrosis of the appendiceal wall, a form of neonatal necrotizing enterocolitis. The similarity between the clinical histories of neonates with so called "appendicitis" and those with necrotizing enterocolitis is pointed out. It is argued that "idiopathic primary peritonitis" probably does not exist, but that the peritonitis may be secondary to similar small perforations of the bowel. The importance of a thorough search for such a perforation is stressed.