Submandibular cystic hygroma resembling a plunging ranula in a neonate. Review and report of a case. Cystic hygromas are large lymphangiomas that are most often found in the posterior triangle of the neck and the axilla in children. They are most frequently found before age 2 and may be massive. After upper respiratory infection, they may become infected and enlarged, causing dysphagia and toxemia. The diagnosis can usually be made by history and physical examination and confirmed by biopsy. Treatment is by surgical excision of small lesions and staged debulking excisions in more severe cases. A patient with a cystic hygroma having many clinical characteristics of a plunging ranula is presented. The cyst fluid was aspirated and analyzed for its amylase, sodium, potassium, chloride, urea nitrogen, glucose, and total protein content. The characteristics of the fluid were also compared with those of lymph and saliva. This report demonstrates the difficulty in determining the diagnosis of a tumor that has the clinical features of a cystic hygroma, as well as a plunging ranula. The necessity of a proper presurgical diagnosis is essential since the form of therapy for each is different and conflicting. A method that distinguishes between the cervical cystic hygroma and a plunging ranula by means of aspirated fluid is discussed. 