Pyriform sinus fistula is a congenital anomaly that develops from the third or fourth branchial cleft, the fistula initially forms in the neck and then develops into a cyst. This condition usually presents as fever in infant and in subjects after childhood, and also presents with similar clinical findings as a painful neck tumor. However, fetal diagnosis has been reported rarely. Here, we report the prenatal diagnosis and the management of delivery based on cytology of the fetal neck cyst in a patient with congenital pyriform sinus fistula. The patient is a 29-year-old (gravia.1 para.0) woman who underwent regular prenatal checkup at our hospital from early pregnancy. A 3 cm cystic mass in the fetal neck and polyhydramnios with amniotic pocket of 9 cm were detected at the gestational age of 28 weeks and four days. Magnetic resonance imaging at the gestational age of 30 weeks and five days indicated the presence of a simple cystic mass (dimension of 25 mm×44 mm×36 mm), between the dorsal epipharynx and left neck, which compressed the trachea at the level of the pharyngo larynx: the mass had low intensity on T1-weighted imaging and high intensity on T2-weighted imaging. As lymphangioma was suspected, fetal neck cyst puncture was performed at the gestational age of 35 weeks and six days to maintain an airway with absolute accurancy and facilitate trial of vaginal delivery. Following aspiration of 26 ml of fluid, the size decreased. Because a cytological examination indicated the presence of a small number of lymphocytes and epithelial cells, congenital cyst of the branchial cleft was diagnosed. Subsequently, the cyst size gradually increased, and hence, vaginal delivery was induced at the gestational age of 37 weeks and four days. The baby weighed 2088g, and had an Apgar score of 8/9. As computed tomography performed after birth indicated the presence of air in the cyst, communication of the cyst with the respiratory tract or esophageal tract was suspected. As the intake of milk by the infant was poor from the first day after delivery and the cyst size increased, we decided to perform radical surgery on the 12th day after delivery, and confirmed the diagnosis of pyriform sinus fistula. Thus, when a unilocular cyst is detected in the fetal neck, the diagnosis of congenital pyriform sinus fistula should be taken into consideration. Moreover, when marked compression of the trachea is noted, the EXIT procedure can be attempted; however, in the current case, vaginal delivery could be selected after fine-needle aspiration. [Adv Obstet Gynecol, 65(3) : 277-282, 2013 (H25.8)]