Cystic hygroma ;s a benign tumor which originates in the lymphatic system. It is found mostly in infants and children under 2 years of age. Its occurrence in adults is rare and a review of the literature revealed that fewer than 70 cases have been reported. In May 1996, a 34 year-old woman came to our clinic with a 10-year history of a slowly enlarging mass on the left side of the neck. Computed tomographic scanning showed a well-defined, homogenous mass. The patient received a total excision of the mass. The pathological diagnosis was cystic hygroma. Currently she is well with no recurrence of disease in the 24 months since the operation. In summary, cystic hygroma is most often seen in infants and children. When a rapidly growing, painless mass on the neck is found in adults, the possibility of cystic hygroma should be considered.