The proliferative potential of two common histologic variants of ameloblastoma was investigated immunohistochemically using Ki-67 antibody on routinely processed, formalin-fixed, paraffin-embedded sections. Thirty cases of ameloblastomas (15 cases of follicular and 15 cases of plexiform type) were analyzed. Autoclave heating pretreatment was employed at 121 degrees C for 20 min prior to analysis. This retrieval method allowed immunoreactive sites of the Ki-67 antigen to become exposed and thus available for immunohistochemical reaction. We found that expressed Ki-67 antigen was localized within the nucleus of tumor cells in both follicular and plexiform ameloblastomas. Immunoreactive cells were localized in the peripheral area of tumor islands as well as in the central stellate reticulum-area. The labeling rate was higher in the plexiform (3.68%) than in the follicular type (1.78%). Results suggest that cell proliferation of ameloblastoma was different depending on histologic variation of the tumor. Further, the proliferative potential was higher in the plexiform ameloblastoma than that in the follicular type.