Using restriction fragment length polymorphism (RFLP) analysis, we demonstrated in 4 of 20 patients with astrocytomas loss of heterozygosity on the short arm of chromosome 17 (17p), in the telomeric segment distal to DNA marker pEW301 (locus D17S58). The loss of heterozygosity may uncover a mutation in a tumour suppressor gene and thus lead to or permit tumour formation. The p53 tumour suppressor gene, which is localized at 17p13, is a likely candidate for the tumour suppressor gene involved. Of the 4 patients with loss of heterozygosity on 17p, one patient had a grade I astrocytoma, another patient had a grade II astrocytoma and 2 patients had glioblastoma multiforme. Since the loss of heterozygosity on 17p was detected in low-grade as well as in high-grade astrocytomas, it is possible that p53 suppressor gene loss may be an early genetic event in the multistep process of astrocytoma formation.