BACKGROUND In animal models of atherosclerosis, augmentation of circulating high-density lipoprotein (HDL) cholesterol exerts a protective effect against development of fatty streaks and promotes plaque regression. RESULTS To investigate the potential of gene transfer to increase HDL cholesterol, a fusion gene encoding human apolipoprotein A-I (apo A-I) under the control of the human cytomegalovirus (CMV) immediate-early promoter was packaged into a recombinant adenovirus (AdCMV apo A-I). BALB/c mice infected with AdCMV apo A-I by intravenous injection accumulate immunoreactive apo A-I in serum; levels 5 days after infection averaged 168 mg/dL. A 35% increase in HDL cholesterol and a 47% increase in total cholesterol were observed in mice infected with AdCMV apo A-I compared with control viruses. Analysis of size-fractionated lipoproteins revealed that human apo A-I is incorporated into murine HDL particles. Expression of human apo A-I declined to < 10% of maximum after 12 days and mRNA encoding apo A-I, prevalent 5 days after infection, was undetectable in the livers of infected mice after 12 days. CONCLUSIONS We conclude that adenovirus-mediated transfer of a gene encoding apo A-I produces transient elevations of circulating HDL cholesterol of a magnitude correlated with important physiological effects. These observations suggest the potential for gene-based therapeutic strategies to reduce cardiovascular risk.