The aim of this study is to evaluate the proliferating activity of ameloblastomas and its correlation to the biological behaviour according to each histological type. 38 cases of solid and unicystic ameloblastomas were reviewed including 1 case of ameloblastic carcinoma and 1 recurrent case. An anti-PCNA antibody, PC10(Dako), was applied for the detection of proliferating cell nuclear antigen (PCNA) in paraffin embedded tissue sections. Also 20 cases of dentigerous cysts were reviewed. In conclusion, we observed that there was no difference between the proliferating activities of the different histological types of solid ameloblastomas. Because 1 case of ameloblastic carcinoma and one recurrent case revealed remarkably high PCNA reactivity, we believe that PCNA would be useful in showing the differentiation between benign and malignant ameloblastomas. In unicystic ameloblastomas, plexiform intraluminal growth was considered to be an important feature in tumorous transformation of cystic epithelium.