Background: The immune response to adenoviral vectors used in gene transfer limits the duration of transgene expression and thus poses a potential limitation to their effectiveness for gene therapy. The need for immunosuppression in transplantation may modify this immune response and facilitate prolonged transgene expression. This study hypothesizes that in the setting of heart transplantation, the use of routine immunosuppression will prolong adenoviral-mediated transgene expression. Methods and results: In a model of rat heterotopic abdominal heart transplantation, 350 μl of viral solution (1 x 109 pfu/ml) was infused into the coronary arteries of donor hearts at the time of procurement. The duration of transgene expression was examined following (a) syngeneic transplantation in non-immunosuppressed animals (group A); (b) syngeneic transplantation in immunosuppressed animals (group B); and (c) allogeneic transplantation in immunosuppressed animals (group C). After transplantation donor hearts were studied at: 1, 4, 8 and 12 weeks. Transgene expression was assessed by histochemical staining of tissue cross sections for β-galactosidase activity. In the non-immunosuppressed syngeneic group (group A), transgene expression bad largely disappeared by 4 weeks, whereas in both the immunosuppressed syngeneic (group B) and immunosuppressed allogeneic (group C) animals expression of the reporter gene persisted for the 12 weeks of the study, although the level of expression decreased significantly over time. Conclusions: This study demonstrates that transgene expression using adenoviral vectors is prolonged by immunosuppression in the heart transplantation setting.