In this paper an ancestral karyotype for primates, defining for the first time the ancestral chromosome morphology and the banding patterns, is proposed, and the ancestral syntenic chromosomal segments are identified in the human karyotype. The chromosomal bands that are boundaries of ancestral segments are identified. We have analyzed from data published in the literature 35 different primate species from 19 genera, using the order Scandentia, as well as other published mammalian species as out-groups, and propose an ancestral chromosome number of 2n = 54 for primates, which includes the following chromosomal forms: 1(a+c1), 1(b+c2), 2a, 2b, 3/21, 4, 5, 6, 7a, 7b, 8, 9, 10a, 10b, 11, 12a/22a, 12b/22b, 13, 14/15, 16a, 16b, 17, 18, 19a, 19b, 20 and X and Y. From this analysis, we have been able to point out the human chromosome bands more "prone" to breakage during the evolutionary pathways and/or pathology processes. We have observed that 89.09% of the human chromosome bands, which are boundaries for ancestral chromosome segments, contain common fragile sites and/or intrachromosomal telomeric-like sequences. A more in depth analysis of twelve different human chromosomes has allowed us to determine that 62.16% of the chromosomal bands implicated in inversions and 100% involved in fusions/fissions correspond to fragile sites, intrachromosomal telomeric-like sequences and/or bands significantly affected by X irradiation. In addition, 73% of the bands affected in pathological processes are co-localized in bands where fragile sites, intrachromosomal telomeric-like sequences, bands significantly affected by X irradiation and/or evolutionary chromosomal bands have been described. Our data also support the hypothesis that chromosomal breakages detected in pathological processes are not randomly distributed along the chromosomes, but rather concentrate in those important evolutionary chromosome bands which correspond to fragile sites and/or intrachromosomal telomeric-like sequences. Copyright © 2005 S. Karger AG, Basel.