We have studied alterations of the p53 gene in 27 patients with malignant gliomas. Loss of heterozygosity (LOH) was investigated by microsatellite analysis in 23 patients (22 informative) and detected in nine. Exons 5 through 9 were amplified by polymerase chain reaction (PCR) in these nine patients: alterations were found in five cases (three missense mutations, one non-sense mutation, and one putative deletion), while in four the DNA sequence was normal. In the four patients where LOH could not be studied, the p53 sequence from tumor DNA was normal. These results indicate that microsatellite analysis is a convenient tool for LOH detection at the p53 locus and that mutations of the p53 gene are present in only part of the patients with LOH, implying the possibility that another tumor suppressor gene is located in the proximity of the p53 locus.