Segmental intestinal dilatation is rare, which causes symptom of bowel obstruction and requires resection. The resection is not only diagnostic but also curative procedure. Recently, author experienced 2 cases of segmental dilatation of the ileum due to focal agenesis of the intestinal muscularis in 7 day & 4 day-old female neonates. The post operative recovery was excellant in the first case after resection of dilated ileum(15cm in length) and end to end anastomosis, and discharged at 20th day. But in the second case, the passage disturbance was not relieved after resection of dilated ileum (30cm in length), and author re-resected 80cm more of dilated proximal ileum at 2 weeks after the first operation. This baby discharged after diarrhea control with Loperin on 1 month after the second operation. Final histologic examination showed 1) normal population of ganglion cells in both narrowed & dilated ileum in both cases. 2) focal abscence of muscularis propria in both cases. 3) relative hypertrophy of inner circular muscle layer and thinned, multiple fragmented outer longitudinal muscle layer in case 2.