Using the major immediate early murine cytomegalovirus (MIEmCMV) promoter to drive expression of beta-galactosidase, we have demonstrated that, following adenoviral-mediated transduction of brain cells in vivo, a single viral infectious unit is capable of producing detectable levels of transgene expression and that gene transfer into the brain is close to 100% efficient. By reducing 100-fold the amount of virus needed to detect large numbers of transduced brain cells, we were able to completely eliminate the cellular inflammation and viral cytotoxicity associated with the delivery of adenoviral vectors into the brain compared to saline-injected controls. These results demonstrate that a strong promoter is necessary to allow the use of low concentrations of adenoviral vectors for gene transfer into the brain, thereby eliminating deleterious side effects and increasing the potential efficacy of gene therapy.