Transitional cell carcinoma (TCC), a neoplasm of urinary bladder urothelial cells, generally appears in either of two forms, papillary non-invasive or invasive TCC, although intermediate forms can occur. Each has a distinctive morphology and clinical course. Altered expression of the p53 and pRb genes has been associated with the more serious invasive TCC, suggesting that the loss of activity of these tumor suppressor proteins may have a causal role in this disease. To test this hypothesis directly, transgenic mice were developed that expressed the simian virus 40 large T antigen (TAg) in urothelial cells under the control of the cytokeratin 19 gene (CK19) regulatory elements. In one CK19-TAg lineage, all transgenic mice developed highly invasive bladder neoplasms that resembled invasive human bladder TCCs. Stages of disease progression included development of carcinoma in situ, stromal invasion, muscle invasion, rapid growth, and, in 20% of affected mice, intravascular lung metastasis. Papillary lesions never were observed. Western blot analysis indicated that TAg was bound to both p53 and pRb, which has been shown to cause inactivation of these proteins. Our findings support suggestions that (i) inactivation of p53 and/or pRb constitutes a causal step in the etiology of invasive TCC, (ii) papillary and invasive TCC may have different molecular causes, and (iii) carcinoma in situ can represent an early stage in the progression to invasive TCC.