The c-mos gene and its protein product mos, components of the mitogen-activated protein kinase transduction pathway, are known to be involved in the control of meiosis and mitosis. Apart from our previous studies on lung carcinomas and astrocytic gliomas, little has been published about its role in human neoplasia. The aim of this study was to investigate the expression of mos in ependymal neoplasms and to correlate it with tumor grade, proliferative fraction, and clinical behavior. We studied mos expression in biopsy specimens from 34 patients with ependymomas. Intracytoplasmic immunopositivity for mos was found in 16 (47%) and was associated significantly with tumor grade: 5 (24%) of 21 grade II ependymomas; 11 (85%) of 13 grade III anaplastic ependymomas (P < .01). Tumors with an MIB-1 labeling index of more than 4% were significantly more likely than those with a lower proliferative fraction to be immunopositive for mos (P = .012). Expression of mos showed a significant negative association with recurrence-free interval (P = .05) but not with overall survival. Our results suggest that overexpression of mos identifies a biologically aggressive subgroup of ependymal tumors and may be involved in their neoplastic progression.