Recurrent chromosome aberrations are associated with many human cancers. Detailed cytogenetic analysis of tumors has benefited enormously from the development of molecular cytogenetic techniques based on fluorescence in situ hybridization (FISH). Comparative genomic hybridization (CGH) is a recently developed FISH technique that allows a rapid and comprehensive identification of imbalanced genomic material in tumour DNA. Comparative genomic hybridisation has been used widely in human medicine to evaluate losses and gains of tumour DNA isolated from a variety of sources, including fresh samples, cell-culture material and archival specimens, and has been instrumental in identifying sites in the human genome which contain genies involved in tumour development and progression. This report describes the first application of CGH in the dog, illustrated by the analysis of DNA isolated from a canine glial tumour cell line.