To evaluate choroid plexus neoplasms for p53 expression.We studied 10 choroid plexus tumors (four papillomas and six carcinomas) by immunohistochemistry using the DO7 anti-p53 monoclonal antibody.Three of four choroid plexus papillomas demonstrated no staining. Scattered nuclear and rare cytoplasmic positivity was present in one papilloma, which showed foci of increased mitotic activity (labeling index 2.5%). Six of six carcinomas were immunoreactive for p53, and three cases had labeling indexes of over 70%. All immunopositive choroid plexus tumors (7/7) exhibited nuclear staining. Punctate cytoplasmic positivity was identified in 5 of 7 cases.Our study suggests that altered p53 expression is detectable by immunohistochemistry in choroid plexus neoplasms and is consistently present in choroid plexus carcinomas.