Ultrasonography is useful in the evaluation of enteric duplication cys
ts in infants. Typically, there is an anechoic fluid-filled lumen with
a double-layered surrounding wall. This case report documents an addi
tional sonographic finding which has not been recorded: peristaltic co
ntractions of the cyst wall. This provides a real-time observation whi
ch is specific in the pre-operative diagnosis of neonatal cystic abdom
inal masses.