Karyotypic data on choroid plexus papillomas are scarce and, to date, have revealed no consistent aberrations. We karyotyped a choroid plexus papilloma which was characterized by a stemline of 52,XX, + 7, + 11, + 12, + 12, + 15, + 18. Additional copies of chromosomes 16, 17, and 20 were also observed in a significant proportion of the metaphase cells analyzed. Based upon this index case, we retrospectively analyzed eight additional choroid plexus papillomas by fluorescence in situ hybridization by using pericentromeric probes to chromosomes 7, 11, 12, 15, 16, 17, 18, and 20. Extra hybridization signals were observed in five of the eight cases examined. All five cases had extra signals with the chromosome 7 probe, four cases had extra signals with the chromosome 12 probe, and three cases had extra signals with the chromosome 15, 17, and 18 probes. The overall DNA content of these same cases (as determined by image analysis) suggests that gains of additional chromosomes other than those examined may be present.