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 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 rel. 27). 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 frequently duplicated in the past or that the entire human genome have duplicated several hundreds of million years ago. Further analysis using the complete genomic sequences of human chromosomes should reveal the chromosomal or genomic duplications, which will gain our knowledge of molecular mechanisms of genome evolution.