Traces of ancient duplications of extensive chromosomal regions are being discovered within the human genomes. For example, the MHC (major histocompatibility complex) gene region on chromosome 6 (6p21.3), which extends at least a few megabases, encodes a cluster of genes that are homologous to those on chromosome 9 (9q33-q34). These regions appeared to have arisen from a regional duplication around the time of vertebrate emergence. However, it is not clear whether such duplications of long chromosomal regions will be found throughout the human genome. To solve this problem, we examined the distribution of homologous genes on human chromosomes. All the human protein-coding genes that have assigned their chromosomal locations are extracted from a DNA database (DDBJ/EMBL/GenBank). By conducting similarity searches among these sequences, we found numerous homologous gene pairs among all of 22 autosomes and two sex-chromosomes. Furthermore, there are many instances that more than one pair of homologous genes is located on a certain pair of chromosomal bands. These findings suggest either that extensive chromosomal regions have been frequently duplicated in the past or that the entire genome of an ancestral vertebrate have been duplicated several hundreds of million years ago, followed by extensive genomic rearrangements. Further analyses using the complete genomic sequences of human chromosomes and comparative analyses of vertebrate genomes should reveal the evolutionary processes of chromosomal or genomic duplications in the course of human evolution.