The aim of this investigation was to corroborate the relationship between DNA ploidy and chromosomal variation in surgically removed colorectal cancers. For 101 specimens from 21 advanced colorectal cancers, the numerical variations in chromosomes 7, 17, and 18 among cells were measured by fluorescence in situ hybridization using DNA probes specific for centromere of each chromosome, and DNA ploidy was determined by laser scanning cytometry or flow cytometry. DNA diploidy (DNA index = 1.0) was linked with minor variation in copy number of chromosomes 7, 17 and 18, whereas DNA aneuploidy (DNA index > or = 1.2) was found exclusively in tumors with large variations in centromere copy number for all chromosomes. There was a significant difference in the degree of intercellular variations in chromosome copy number between diploid and aneuploid clones for all chromosomes examined (P < 0.001). In near-diploid clones (1.0 < DNA index < 1.2), the numerical variation of chromosome 18 was significantly different from that in diploid clones (P < 0.002), but it was not different from that in aneuploid clones. These observations support the hypothesis that chromosomal instability is associated with DNA aneuploidy in colorectal cancers. Additionally, they suggest that near-diploid tumors are also unstable at a lower level than classic aneuploid tumors and that all chromosomes are not affected equally in near-diploid cases.