The development and application of a procedure for interphase cytogenetics on brain tumor material is described. Nuclei isolated from freshly removed brain tumor tissue were investigated for chromosomal aberrations by nonradioactive in situ hybridization with a panel of chromosome-specific probes. The panel consisted of nine satellite DNA probes specific for the centromeric regions of chromosomes I, 6,7, 10, I I, 17, 18, X, and Y. For each probe, the number of hybridization signals per cell was determined in 200 nuclei. It was inferred from the hybridization results that in I I gliomas (seven astrocytomas grade 11-IV, three oligodendrogliomas, and one ependymoma) the numerical aberrations were gains of chromosomes I (once), 7 (twice), 10 (once), I I (twice), and X (twice); losses of chromosomes I (once), 10 (twice), 17 (twice), and Y (once); and complete tetraploidy (once). Among the I8 investigated meningiomas monosomy I8 and trisomy I7 were observed once and twice, respectively. An additional hybridization with a cosmid probe for the BCR gene on 22q I I indicated monosomy 22q in I I meningiomas. These results show the value of interphase cytogenetics for the analysis of solid tumors for which it is relatively difficult t o obtain sufficient metaphases of good quality for conventional cytogenetics.