Adenomatoid odontogenic tumor is characterized histologically by abundant epithelial parenchymal cells, containing ductlike structures surrounded by cylindric cells.In this article we report on the experience of a case of this tumor. The patient is a female, 12 years old, from the end of February 1982 she noticed a swelling of the gingiva at 2-4 portion but left it alone at first. As the swelling did not disappear, she consulted a certain dentist, who introduced her to our clinic. Oral examination revealed that| 23 were unerupted, a walnut-size, relatively clearly bordered swelling was shown at the labial side of gingiva at 2-4 portion and palpated like parchment in some parts. The surface was bluish without oppressive pain. X-ray examination showed that| 23 were impacted and a monolocular radiolucent area was noted containing the tooth crown of 2, and 5| 5 was congenitally missing. On 2 April the tumor was enucleated together with impacted| 2 under general anesthesia.Histologically the tumor parenchyma was composed of cells similar to the enamel organ, showing ductlike structures in the parenchyma, composed of ovoid or short cylindric cells, as well as small calcifications. Histopathologically this case showed the initial developmental stage of adenomatoid odontogenic tumor. Further, the tumor parenchyma appeared and proliferated continuously with the epithelium of cystic wall, a transition picture was shown between the parenchyma and the epithelium of cystic wall. This tumor was considered to be follicular cystic origin.