Microsatellite instability (MSI) is a form of genomic instability in tumors that may reflect mechanisms underlying carcinogenesis. Assessment of MSI in various types of sporadic tumors is therefore relevant to an understanding of molecular pathogenesis. In the case of sporadic adult gliomas, destabilization of mononucleotide, dinucleotide, and longer repeat sequences has been reported in high-grade tumors, though published estimates of the frequency of MSI vary widely. In the present work, we quantitated the frequency of length alterations at three microsatellite loci in 26 glioma/normal tissue pairs and at nine additional loci in 16 of the pairs. We analyzed di- and tetranucleotide markers, including five previously reported to be unstable in gliomas. and examined mostly high-grade tumors, both diploid and aneuploid. A large proportion of the tumor and normal brain specimens had no detectable activity of the DNA repair protein O6-methylguanine-DNA methyltransferase, a prevalent phenotypic trait in these tissues that we thought might be associated with MSI. We observed no length alterations in 222 sequence analyses, and estimate the frequency of MSI in our tumor sample as < 0.45% unstable sequences among all sequences examined, or < 3.9% gliomas with unstable sequences. We conclude that microsatellite length alterations are infrequent in our tumor population, and interpret currently available literature to indicate that the frequency of MSI is low in sporadic adult gliomas.