A hidrocystoma which originated in a mammarylike gland (MLG) of the vulva is described. It involved the main collecting duct and was characterized by a peripheral basal layer of myoepithelium and a luminal layer of cuboidal to columnar cells with discrete cytoplasmic snouts and by lobules opening into the cavity. Adhering to it were cutaneous glands, including MLGs, and dartos muscle. These features distinguish it from the other cysts of the vulva, including vestibular, urogenital sinus, and keratinous cysts. Another cystic lesion of the MLGs was evidenced only microscopically and involved multiple whole glands. It resembled similar alterations in eccrine, apocrine, and mammary glands, and most likely represented involution cysts of MLGs.