A 2,510 g female newborn was delivered by Cesarean section at 33 weeks gestation due to increasing volume of her huge bladder and bilateral hydronephrosis. Just after birth 190 ml of urine was drainaged by catheterization without difficulty. Voiding cystourethrography showed no VUR. She had no ureteral dilation, either. Urodynamic study revealed large bladder capacity and detrusor hypocontractility. Neither neurological nor gastrointestinal abnormality was detected neonatally. Clean intermitted catheterization was performed every 4 hours. At 12 months of her age she had no history of urinary tract infection. However, as she grew up with normal diet, severe constipation became apparent. This is a case of idiopathic intestinal pseudo-obstruction syndrome whose gastrointestinal symptom was masked in the newborn period. Attention should be paid for gastrointestinal tract as well as urinary tract when the body has congenital megacystis with unknown etiology.