BACKGROUND AND AIMS: The relationships between very high plasma HDLc and subclinical atherosclerosis are still a matter of debate. METHODS AND RESULTS: Twenty subjects with primary hyperalphalipoproteinemia (HAL, with HDLc in the highest 10th percentile and absence of overt secondary causes of this condition), aged 30-65 years, were compared with 20 age and sex-matched controls. Lipid determination, lipoprotein particle distribution (Lipoprint(®)), Cholesterol Efflux Capacity (CEC), plasma adhesion molecule, analyses of CETP, SRB1 and LIPG genes and of different markers of subclinical vascular disease (ankle-brachial index, ABI; carotid intima-media thickness, cIMT; brachial-artery flow mediated dilation, FMD) were performed. Fasting HDLc levels were 40 mg/dl higher in HAL subjects while LDLc concentration was comparable to control group. CETP gene analysis in HAL subjects identified one novel rare Single Nucleotide Polymorphism (SNP, Asp131Asn), possibly damaging, while the common SNP p.Val422Ile was highly prevalent (50% vs. 27.4% in a control population). No rare mutations associated with HAL were found in SR-B1 and LIPG genes. Polyacrylamide gel electrophoresis in HAL subjects disclosed larger and more buoyant HDL particles than in controls, while LDL profile was much more similar. ABI, cIMT and arterial plaques did not differ in cases and controls and the two groups showed comparable FMD at brachial artery examination. Similarly, ABCA1 and ABCG1 HDL-mediated CEC, the most relevant for atheroprotection, did not discriminate between the groups and only ABCG1 pathway seemed somewhat related to arterial reactivity. CONCLUSIONS: HDL dimension, function and genetics seem scarcely related to subclinical atherosclerosis and vascular reactivity in middle-aged HAL subjects.