The long-term effect of elevated levels of human apolipoprotein AI (apoAI) on atherosclerosis was assessed using human apoAI transgenic mice on a double mutant LDL receptor-deficient (LDLr-/-) and mouse apoAI-deficient (apoAI-/-) background. When they were fed a high fat diet, atherosclerosis in transgenic human apoAI, LDLr-/-, apoAI-/- mice (huapoAITg) was compared with LDLr-/- mice that expressed normal amounts of apoAI (msapoAI) or LDLr-/- mice that lacked mouse apoAI (noapoAI). The atheroprotective effect of human apoAI was demonstrated by a greater than six-fold inhibition in lesion areas in the aortic wall and heart valves compared to the two control strains after 27 or 36 weeks. Plasma apoAI concentrations in huapoAITg mice were considerably higher than in msapoAI mice (600 and 37 mg/dL, respectively). The human apoAI transgene led to several plasma HDL subpopulations, with high levels of prebeta-HDL and a significant decrease in total plasma cholesterol. This was observed without a change in total HDL cholesterol levels. Thus, elevated levels of human apoAI in LDL receptor-deficient mice lacking mouse apoAI conferred profound protection against diet-induced over extended periods of time.