Cyclooxygenase (COX)-2, the inducible isoform of prostaglandin H synthase, has been implicated in the growth and progression of a variety of human cancers. Although COX-2 overexpression has been observed in human gliomas, the prognostic or clinical relevance of this overexpression has not been investigated to date. In addition, no study has analyzed the relationship between COX-2 expression and other molecular alterations in gliomas. Consequently, we examined COX-2 expression by immunohistochemistry in tumor specimens from 66 patients with low- and high-grade astrocytomas and correlated the percentage of COX-2 expression with patient survival. We also analyzed the relative importance of COX-2 expression in comparison with other clinicopathological features (age and tumor grade) and other molecular alterations commonly found in gliomas (high MIB-1 level, p53 alteration, loss of retinoblastoma (Rb) protein or p16, and high bcl-2 level). Kaplan-Meier analyses demonstrated that high COX-2 expression (>50% of cells stained positive) correlated with poor survival for the study group as a whole (P < 0.0001) and for those with glioblastoma multiforme in particular (P < 0.03). Cox regression analyses demonstrated that COX-2 expression was the strongest predictor of outcome, independent of all other variables. In addition, high COX-2 expression correlated with increasing histological grade but did not correlate with positive p53 immunostaining, bcl-2 expression, loss of p16 or retinoblastoma protein expression, or high MIB-1 expression. These findings indicate that high COX-2 expression in tumor cells is associated with clinically more aggressive gliomas and is a strong predictor of poor survival.