Alterations in the tumor suppressor gene p53 lead to impaired cell cycle control, allowing for the development and growth of tumors. To restore a loss of p53 function, we performed a phase I study of intratumoral gene therapy with adenovirus expressing wild-type p53 in patients with non-small cell lung cancer carrying mutations in the p53 gene. Furthermore, in a phase II study, gene therapy was complemented with simultaneous cisplatin/vinorelbine treatment. Biopsies were obtained from all treated patients before and 24-48 hours after gene therapy to study changes in the expression of p53 target genes. We report here that in most of the cases, the target gene p21 was up-regulated, especially when injection of higher doses of p53-expressing adenovirus was combined with simultaneous chemotherapy, whereas Pig3, previously reported to be highly up-regulated by p53, generally did not show a clear increase. Interestingly, a clear p21 gene response was observed only in tumors showing stabilization or regression. We conclude that p21 appears to be up-regulated after adenovirus-mediated p53 gene transfer and is the most sensitive marker tested for biological response to gene therapy in the small cohort of non-small cell lung cancers that were studied.