Mutations of the tumor suppressor gene p53 are common in bladder cancer. To determine whether p53 gene transfer would lead to decreased viability of bladder cancer cells, we studied the effect of p53 gene transfer in human bladder cancer cell lines with either mutant or wild-type p53. Bladder cancer cell lines 5637 and J82 (which express only mutant p53) and 253J-BV (which expresses wild-type p53) were transduced with vectors containing the beta-galactosidase gene (Ad5-lacZ), wild-type human p53 gene (Ad5CMV-p53), or no foreign gene (DL312 or Ad5-polyA). X-gal staining of cells exposed to Ad5-lacZ showed that the adenoviral vector was capable of transducing each of the cell lines. Increases in p53, p21(waf1/cip1) and bax protein were demonstrated following exposure to Ad5CMV-p53, and there was a dose-dependent increase in the number of apoptotic cells. Cell viability was decreased in all three cell lines, although J82 was less sensitive than either 5637 or 253J-BV. To determine whether cisplatin increases sensitivity of J82 cells to Ad5CMV-p53, we performed median effect analysis for cisplatin combined with Ad5CMV-p53 or DL312. The combination index for cisplatin plus Ad5CMV-p53 revealed synergy, whereas cisplatin and DL312 were only additive. These results suggest that forced p53 gene expression is cytotoxic to human bladder cancer cells with either p53 mutant or wild-type background, and that combination with cisplatin is a potential method for overcoming resistance.