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A nationwide survey on neonatal surgery conducted by the Japanese Society of Pediatric Surgeons has demonstrated that the mortality of neonatal intestinal perforation has risen over the past 15 years. The incidence of intestinal perforation in extremely low–birthweight (ELBW) neonates has been increasing as more ELBW neonates survive and as the live–birth rate of ELBW has increased. In contrast to necrotizing enterocolitis (NEC) and focal intestinal perforation (FIP), the pathogenesis of meconium–related ileus, defined as functional bowel obstruction characterized by delayed meconium excretion and microcolon, remains unclarified.The histology of 13 ELBW neonates with intestinal perforation secondary to meconium–related ileus was reviewed, and the radiology of 33 cases of meconium–related ileus diagnosed on contrast enema was reviewed. Specimens obtained from 16 ELBW neonates without gastrointestinal disease served as age–matched controls for histological assessment.The size of the ganglion cell nucleus in meconium–related ileus and in control subjects was 47.3 ± 22.0 μm2 and 37.8 ± 11.6 μm2, respectively, which was not significantly different. In all cases of meconium–related ileus, contrast enema demonstrated a microcolon or small–sized colon, with a gradual caliber change in the ileum and filling defects due to meconium in the ileum or colon, showing not–identical locations of caliber changes and filling defects.Morphological immaturity of ganglia was not suggested to be the pathogenesis of meconium–related ileus. Impaction of inspissated meconium is not the cause of obstruction, but the result of excessive water absorption in the hypoperistaltic bowel before birth, although the underlying mechanism responsible for the fetal hypoperistalsis remains unclear.