Previous studies have shown that the negative cell cycle regulator WAF1/Cip1 is often overexpressed in human gliomas and that WAF1/Cip1 overexpression may be a factor in cancer chemoresistance. We established a doxycycline-inducible WAF1/Cip1 expression system in two glioblastoma cell lines and examined the role of WAF1/Cip1 in their response to the chemotherapy agents 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) and cis-diamminedichloroplatinum (cisplatin), in an isogeneic background. Our results showed that the induction of WAF1/Cip1 expression rendered glioma cells resistant to cell death induced by BCNU and cisplatin. Using an in vivo host-cell reactivation DNA repair assay, we demonstrated that WAF1/Cip1 enhances the repair of BCNU-induced DNA damage. We conclude that WAF1/Cip1 allows repair of BCNU- and cisplatin-damaged DNA and protects glioma cells from chemotherapy agent-induced apoptosis. Thus, blocking WAF1/Cip1 production or function may serve as a useful chemosensitization regimen for glioma.