In order to study the possible biological differences between anaplastic and typical seminoma, the following factors were studied in 11 cases of anaplastic seminoma and 15 cases of typical seminoma: mitotic activity, proliferating cell nuclear antigen (PCNA) expression, immunohistochemical analyses for cytokeratin, vimentin, placental alkaline phosphatase (PLAP), beta-human chorionic gonadotropin (beta-hCG), alpha-fetoprotein (AFP) and c-myc oncoprotein. Anaplastic seminoma was classified according to Mostofi's criteria, which is primarily based on the mitotic activity of the tumor. Mitotic activity was evaluated by both mitotic count and rate. Statistically significant correlations were observed between mitotic count and mitotic rate (R = 0.891), and between the mitotic count and PCNA labeling index (R = 0.792), in both typical and anaplastic seminomas. Immunostaining patterns for cytokeratin, vimentin, PLAP, beta-hCG, AFP and c-myc oncoprotein were not significantly different between typical and anaplastic seminoma. The present data indicated that no apparent clinicopathologic and immunohistochemical parameters discerning anaplastic seminoma from typical seminoma were present, when identifying anaplastic seminoma on the basis of high mitotic count. Anaplastic seminoma may therefore simply represent seminoma with high proliferative activity.