Ameloblastoma is a benign odontogenic tumor with an aggressive biological behavior, and the surgical treatment frequently results in failure for the postoperative recurrence. The aim of this article was to investigate whether the proliferative ability and prognosis of ameloblastoma could be evaluated by the radiographic boundary. The ameloblastoma cases treated by the conservative therapy in our hospital between 1981 and 2001 were divided into three groups based on the nature of the radiographic borders of the lesions. The biologic behavior was evaluated by Ki-67 antibody immunohistochemically. Comparisons of prognosis and Ki-67 expression were carried out by statistic methods. There were 24 cases of well-defined edge with sclerosis (group I), 41 cases of well-defined edge without sclerosis (group II) and 32 cases of ill-defined edge (group III). The recurrent rates were 29.2% in group I, 43.9% in group II and 62.5% in group III (P<0.05). The cells in group III expressed the highest Ki-67 level (P<0.05). The radiographic boundary could be used as one of indicators in evaluating the proliferative ability of ameloblastoma and the patient's prognosis, which was consistent with Ki-67 expression.