OBJECTIVE This study evaluated the effectiveness of marsupialization for the treatment of cystic ameloblastoma. METHODS Clinical and histologic examination of 24 cystic ameloblastomas were performed before and after marsupialization. RESULTS Clinically and radiographically, the effect of marsupialization was evaluated as extremely effective (five cases), effective (11 cases), and noneffective (eight cases). Marsupialization was most effective in teenaged patients. In three cases, the tumor disappeared macroscopically, and further surgery was not done. Histologically, the growth characteristics of the tumor were classified into expansive and invasive types. Cytologic pattern was classified into three types according to the outer cell layer of the tumor parenchyme: cuboidal cell, columnar cell, and basal cell. Premarsupialization, the expansive pattern with the cuboidal cell type was dominant, whereas for postmarsupialization many cases that had been expansive changed to an invasive type showing columnar or basal cell types. CONCLUSIONS Marsupialization was found to be useful as a preliminary treatment of the cystic ameloblastoma. However, it must be borne in mind that following marsupialization the tumor still has a potential to infiltrate into the surrounding tissues.