Bladder cancer is one of the most common adult malignancies world-wide. In the United Kingdom, it accounts for 12 700 new cancer cases per annum and 5300 deaths. Fluorescence in situ hybridization has been applied to study both numerical chromosomal aberrations and small alterations which can be detected using large insert genomic DNA clones as probes. The tumor suppressor genes now comprise a large group of genes whose inactivation via genetic or other mechanisms contributes to the development of cancer. Several oncogenes are implicated in transitional cell carcinomas development, progression and these contribute to tumor development in a dominant manner via overexpression of the normal gene product or expression of a mutant gene product with altered function. The repertoire of known genes and loci must be examined in much greater detail both in terms of the function of the genes concerned and their impact on the clinical behavior of tumors.