Owing to its central role in multiple cellular functions, p53 is an attractive candidate for gene replacement therapy. We studied the role of adenovirus-mediated p53 gene (p53Ad) therapy on sensitivity of two ovarian cancer cell lines, OVCAR-3 (p53(mut)) and SK-OV-3 (p53(wt)), to docetaxel, CPT-11 and SN-38 exposures. Expressions of Bcl-XL, Bcl-XS, p53, Gadd45, c-fos, p21(waf1/cip1), Bax, Bcl-2 and Mcl-1 were measured after concomitant p53Ad and drug exposures. In SK-OV-3 cells containing a normal p53 gene, p53Ad alone or concomitantly with docetaxel, CPT-11 or SN-38 exposures did not have an effect on cell growth, cell cycle distribution or induction of apoptosis. In OVCAR-3 cells, p53 gene therapy inhibited the cell growth and sensitized cells to CPT-11/SN-38, but not to docetaxel. Growth inhibition and sensitization were results of G2M cell cycle arrest and increased apoptosis. In SK-OV-3 cells, but not in OVCAR-3 cells, CPT-11/SN-38 exposures alone increased p21(waf1/cip1) expression. The p53Ad therapy induced strong p21(waf1/cip1) expression in both cell lines. In addition, the expression of Bax and expression ratios Bax/Bcl-2 and Bax/Bcl-XL increased in p53Ad-infected OVCAR-3 cells, but not in SK-OV-3 cells. These expression ratios were further increased in p53Ad+CPT-11/SN-38-exposed OVCAR-3 samples. These results support the combination of p53 gene therapy with topoisomerase I inhibitors SN-38/CPT-11 when tumour cells contain mutated p53. When p53 status is normal, p53 gene therapy is not effective alone or concomitantly with CPT-11/SN-38. Increased expression ratios of Bax/Bcl-2 and Bax/Bcl-XL might serve as positive markers for effective p53 gene therapy and concomitant topoisomerase I inhibitor therapy.