Choroid plexus neoplasms account for less than 1% of all intracranial tumors, with papillomas (CPPs) more frequent than carcinomas (PCPs). Immunocytochemical characterization of these neoplasms has been limited. Glial fibrillary acidic protein (GFAP), S100 protein, and keratin have been variably demonstrated by others. Ten cases were identified at two hospitals over a 25-year peroid; six were children and four were adults. There were seven cases of CPP and three of CPC. Extracranial metastases occurred in one case of CPC and multiple local recurrences were common. Immunohistochemical examination was performed with polyclonal antibodies to keratin, alpha-fetoprotein (AFP), desmin, neurofilament, glial fibrillary acidic protein (GFAP), neuron-specific enolase (NSE), and S100 protein, and with monoclonal antibodies to vimentin, 45- to 54-kd cytokeratin (CKER), and carcinoembryonic antigen (CEA). Among the seven cases of CPP, five were positive for CKER, three for keratin, two for CEA, two for NSE, and five for S100. Three cases of CPC were positive for CEA, three for CKER, and two for keratin. With one exception, when a neoplasm was positive for CEA and S100 it was also positive for CKER. Positivity for CEA in this group was associated with more aggressive histologic pattern and heralded a worse prognosis. S100 immunoreactivity appeared to predominate in well-differentiated neoplasms. Keratin and CKER were found in both CPP and CPC, but may be useful in the distinction from ependymomas. Statistical analysis resulted in the following classification rule: If the CEA stain is positive and the S100 stain is negative, then the tumor is malignant; otherwise, the tumor is benign.