The objective of this study was to investigate whether activity and protein expression of ornithine decarboxylase (ODC) the metabolism-rate limiting enzyme of the polyamine biosynthesis, which is involved in the regulation of cellular growth and differentiation, reflects the distinct histopathologic characteristics indicative of atypia and anaplasia in meningiomas as well as their impact in recurrences. The authors previously evaluated its value as a critical factor of tumor development in various brain tumors. Among 152 meningiomas, World Health Organisations (WHO) grade I meningiomas (n = 121) exhibited a low ODC activity, while meningiomas WHO grade II (n = 22) and WHO grade III (n = 9), respectively, showed a significantly increased ODC activity. Recurrent WHO grade I meningiomas exhibited the same low enzyme activity and immunohistochemical staining index for ODC positive tumor cells as their primary tumors, whereas raising ODC activity and protein expression in recurrent meningiomas paralleled malignant transformation. These results indicate that ODC reflects aggressive growth and malignization in meningiomas. Especially in recurrent meningiomas, the determination of its activity and protein expression by immunohistochemistry may provide a useful diagnostic tool to recognize malignant progression.