Hydrocolpos, found chiefly in the new-born, characteristically presents as a mass in the lower abdomen associated with a cystic bulge at the introitus or an imperforate hymen. There is a membranous obstruction at the vaginal outlet in conjunction with excessive secretion of the glands of the infantile uterine cervix. A vaginogram made by injecting the cyst with radiopaque material helps to confirm the diagnosis. The best treatment is simple incision of the occluding vaginal diaphragm. In 6 of the 40 cases described in the literature since 1940, hysterocolpectomy was done, and the diagnosis was made only after examination of the removed surgical specimen. In 3 of the 4 cases reported by the author the condition occurred in newborn babies; in the fourth it became manifest at the age of 2 months.