A cystic hygroma is an abnormal growth that usually appears on a baby's neck or head. It consists of one or more cysts and tends to grow larger over time. The disorder usually develops while the fetus is still in the uterus, but can also appear after birth.

Also known as cystic lymphangioma and macrocystic lymphatic malformation, the growth is often a congenital lymphatic lesion of many small cavities (multiloculated) that can arise anywhere, but is classically found in the left posterior triangle of the neck and armpits. The malformation contains large cyst-like cavities containing lymph, a watery fluid that circulates throughout the lymphatic system. Microscopically, cystic hygroma consists of multiple locules filled with lymph. Deep locules are quite big, but they decrease in size towards the surface.

Cystic hygromas are benign, but can be disfiguring. It is a condition which usually affects children; very rarely it can be present in adulthood.

Currently, the medical field prefers to use the term lymphatic malformation, because the term cystic hygroma means water tumor. Lymphatic malformation is more commonly used now because it is a sponge-like collection of abnormal growth that contains clear lymphatic fluid. The fluid collects within the cysts or channels, usually in the soft tissue. Cystic hygromas occur when the lymphatic vessels that make up the lymphatic system are not formed properly. The two types of lymphatic malformations are macrocystic (large cysts) and microcystic (small cysts) lymphatic malformations. A person may have only one kind of the malformation or can have a mixture of both macro- and microcysts.

Cystic hygroma can be associated with a nuchal lymphangioma or a fetal hydrops. Additionally, it can be associated with Down syndrome, Turner syndrome, or Noonan syndrome. If it is diagnosed in the third trimester, then chances of association with Down syndrome are increased, but if diagnosed in the second trimester, then it is associated with Turner syndrome.

A lethal version of this condition exists, known as Cowchock–Wapner–Kurtz syndrome, that, in addition to cystic hygroma, includes cleft palate and lymphedema, a condition of localized edema and tissue swelling caused by a compromised lymphatic system.