Comparative genomic hybridization (CGH) allows rapid screening for DNA copy number gains and losses across the entire genome. CGH analyses have revealed a number of common aberrations and characteristics associated with specific tumor cells or pathogeneses. Recurrent aberrations suggest that tumor-related gene(s) may be located in such regions. Furthermore, some specific aberrations may serve as novel diagnostic features. Quantitative chromosomal analyses based on CGH have also provided stimulating information associated with chromosomal instability, genetic pathways, telomerase activity status, and DNA ploidy and have yielded valuable insights into tumor pathology. This review focuses on scientific advances facilitated by this technique.