Sixty-eight neonates with functional ileus were reviewed. Twelve required laparotomy; in seven, histological studies revealed decreased ganglia and ganglion cells of the myenteric plexus (MP) (Group A), and in five, MP was normal (Group B). In the remaining 56 cases, obstructive symptoms were relieved following conservative therapy (Group C). All Group A cases except one had normal birth weight, while Group B and C cases showed significantly lower birth weights. A marked caliber change of the small intestine and/or small-caliber distal intestine with meconium stagnation in the proximal intestine was commonly demonstrated at operation in Group A and B, or on contrast enema in Group C. Four Group A cases died of enteritis, and three survivors suffered from prolonged obstructive symptoms. The grade of histological abnormality of MP correlated with the clinical outcome. In Group B, three died of sepsis shortly after surgery, but two survivors have been free from symptoms. Group A can be categorized as Hirschsprung's disease-allied disorders (HAD). Group B and C can be categorized as meconium-related ileus (MRI). The similarity of the macroscopic findings of HAD and MRI, and the occurrence of MRI exclusively in low birth weight neonates, strongly suggest that functional immaturity of MP plays a role in the etiology of MRI.