An inhibitor of telomerase activity maps to chromosome 10p15.1. A series of 51 high-grade gliomas was analyzed for loss of heterozygosity (LOH) on chromosome 10 and for telomerase activity. In univariate analysis, LOH10p (59%) and LOH10q (61%) were associated with telomerase activity (55%; p < 0.0001 and p = 0.0006). In multivariate analysis, only LOH10p remained statistically related to telomerase activity, suggesting that the telomerase repressor gene located on 10p15.1 is inactivated in high-grade gliomas.