Neonatal hydrometrocolpos is the cystic dilatation of the vagina and uterus, which is caused by the combination of vaginal obstruction such as imperforate hymen, transverse vaginal septum, or vaginal atresia, and glandular secretion by maternal estrogenic stimulation. Although hydrometrocolpos is a rare congenital abnormality unlike pubertal hematocolpos, it is one of the relatively common abdominal masses in neonates. In typical cases the diagnosis may be determined easily by the combination of a pelvic mass, upper urinary tract dilatation and a bulging membrane in the vaginal introitus. The presense of a lower abdominal mass in a female infant should always arouse suspicion of hydrometrocolpos and lead to careful examination of the vagina. We report a case of neonatal hydrometrocolpos due to imperforate hymen which was initially presented as a large abdominal mass and a bulging membrane in the vaginal introitus. For 72 hrs, the abdominal mass increased rapidly, accompanied by urinary tract obstruction. It was relieved by a simple incision of the imperforate hymen and drainage of 300 cc of whitish mucoid vaginal fluid, and by Foley catheter drainage of 750 cc urine. No other anomaly was observed.