A new molecular cytogenetic method, comparative genomic hybridization(CGH), was reviewed. CGH produces a map of DNA sequence copy number on a normal metaphase spread after hybridization with the mixture of tumor DNA and normal reference DNA, which are detected with different fluorochromes, respectively. Then the ratio of two fluorochromes are analysed with digital image analyzer and reveals a real copy number of the DNA sequences. CGH surveys entire chromosomes at a time and would clarify oncogenes and tumor suppressor genes, which had not known.