We undertook a cytogenetic analysis of 29 human brain tumors using double-target fluorescence in situ hybridization (FISH) and focusing on chromosome arm 1p. One or more tumor suppressor genes in this arm have been suggested to be important in a variety of neuroectodermal tumors. The series included 9 oligodendrogliomas, 4 mixed gliomas, 10 astrocytomas, 4 glioblastomas, and 2 central neurocytomas. We hybridized pericentromeric (1q12) and subtelomeric (1p36) DNA probes to cell nuclei prepared from paraffin-embedded tissues and observed a strikingly high incidence of deletion of at least part of 1p in oligodendrogliomas (100%) and mixed gliomas (75%). The results of the FISH analyses were confirmed by demonstration of loss of heterozygosity for a microsatellite polymorphism in 10 of the 29 tumors. As well as supporting the feasibility of FISH for detecting allelic deletions in chromosomes from paraffin-embedded tumor samples, the alteration of 1p reported here will contribute to an understanding of the molecular genetic events in oligodendroglial tumor development.