The cytological nature and proliferative activity of bizarre neoplastic cells, widely scattered in pleomorphic adenomas of salivary gland origin were studied. Pleomorphic adenomas containing numerous bizarre neoplastic cells were found in four cases, and were equal to 2.9% of all pleomorphic adenomas examined. All four cases presented as well-circumscribed, firm masses measuring less than 1.5 cm in size, located in the palate, and were of 7 months to 4 years duration. Histopathologically, these pleomorphic adenomas were cell rich type, and were well demarcated from surrounding tissues, although their fibrous capsules were partially defective. In addition to characteristic histopathological findings of pleomorphic adenoma, numerous neoplastic cells with bizarre appearance were scattered throughout the lesion, excepting for tubuloductal structures. These bizarre neoplastic cells had irregular-shaped and large nuclei with or without hyperchromatism, although their nucleoli were small and mitotic figures were few. Furthermore, there were many multinucleated giant cells, some of which showed multilobulated nuclei. Neither necrosis nor infarct was seen in the tumors. Immunohistochemically, bizarre neoplastic cells scattered in solid-proliferating areas and myxoid areas were neoplastic myoepithelial cells in nature. There was no statistical significance of MIB-1 labeling indices between pleomorphic adenomas with bizarre neoplastic cells and usual pleomorphic adenomas. The p53 labeling indices were quite low. Although the benign nature of pleomorphic adenomas with numerous bizarre neoplastic cells and hypercellularity, distinguishing such pleomorphic adenomas from various stages of malignant transformation in pleomorphic adenomas and other carcinomas should be made by histological section of submitted biopsy specimen or aspirated content for cytological diagnosis. The present paper suggests that the term 'bizarre cell pleomorphic adenoma' is an appropriate name for this neoplasm, in that it is distinguished from the usual benign pleomorphic adenoma which is easily diagnosed by routinely prepared histological or cytological stainings.