Recent molecular studies on transitional cell carcinomas (TCC) of the urinary bladder suggest two genetically different tumor types with a varying degree of genomic instability-pTa (grade 1 and 2) tumors are usually "genetically stable". These tumors have rarely p53 alterations and contain only few genomic alterations (deletions, DNA sequence copy number gains, amplifications). Genomic changes in these tumors typically include losses of the chromosomes Y and 9 as well as 1q gains. TCC's with invasive growth pattern (pT1-4) are mostly "genetically instable". They have p53 alterations in about 50% of tumors, and often numerous DNA sequence copy number changes including deletions of 2q, 5q, 6q, 8p and 11p as well as gains at 1q, 3p, 3q, 5p, 6p, 8q und 10p. High level amplifications at various sites of the genome are also frequent in these "instable tumors". They may pinpoint the localization of overexpressed oncogenes. Further studies will have to investigate whether genetic analyses can add useful information to the currently used pathological (pTNM) and clinical (superficial/invasive) classifications of urinary bladder cancer.