Background— Carriers of the apolipoprotein A-I _Milano (apoA-I _M ) mutant have very low plasma high-density lipoprotein cholesterol (HDL-C) levels but do not show any history of premature cardiovascular disease or any evidence of preclinical vascular disease. HDL is believed to prevent the development of vascular dysfunction, which may well contribute to HDL-mediated atheroprotection. Whether the low HDL level of apoA-I _M carriers is associated with impaired vascular function is presently unknown. Methods and Results— The vascular response to reactive hyperemia, assessed by measuring postischemic increase in forearm arterial compliance, and the plasma concentration of soluble cell adhesion molecules were evaluated in 21 adult apoA-I _M carriers, 21 age- and gender-matched nonaffected relatives (control subjects), and 21 healthy subjects with low HDL-C (low-HDL subjects). The average plasma HDL-C and apoA-I levels of apoA-I _M carriers were remarkably lower than those of control subjects and significantly lower than those of low-HDL subjects. The postischemic increase in forearm arterial compliance in the apoA-I _M carriers was 2-fold greater than in low-HDL subjects and remarkably similar to that of control subjects. Plasma soluble cell adhesion molecule levels were similar in apoA-I _M carriers and control subjects but were greater in low-HDL subjects. When incubated with endothelial cells, HDL isolated from apoA-I _M carriers was more effective than HDL from control and low-HDL subjects in stimulating endothelial nitric oxide synthase expression and activation and in downregulating tumor necrosis factor-α–induced expression of vascular cell adhesion molecule-1. Conclusions— Despite their very low HDL levels, apoA-I _M carriers do not display typical features of impaired vascular function because of an improved activity of apoA-I _M HDL in maintaining endothelial cell homeostasis.