BACKGROUND Cholesterol- overloaded high- density lipoprotein ( HDL) particles exert a negative impact on the antiatherogenic function of HDL in experimental studies. However, it remains unclear whether cholesterol- overloaded HDL particle is involved in the development of atherosclerosis in humans. OBJECTIVES The objective of this study was to explore whether cholesterol- overloaded HDL particles are associated with the progression of carotid atherosclerosis in a cardiovascular disease- free population. METHODS Baseline HDL particle number was measured using nuclear magnetic resonance spectroscopy in 930 participants ages 45 to 74 years in a community- based cohort study. An estimate of cholesterol molecules per HDL particle ( HDL- C/ P ratio) was calculated as the ratio of HDL cholesterol to HDL particles. HDL- C/ P ratio was categorized as < 41.0 ( lowest), 41.0 to 46.9, 47.0 to 52.9, and $ 53.0 ( highest) using a fixed increment method. Modified Poisson regression was used to assess the association between HDL- C/ P ratio and 5- year progression of carotid atherosclerosis as indicated by progression of carotid plaques and change in total plaque area ( TPA). RESULTS Mean baseline HDL- C/ P ratio was 46.4 9.3 ( range 23.8 to 86.9). Baseline HDL- C/ P ratio was significantly associated with 5- year progression of carotid atherosclerosis. Participants with the highest HDL- C/ P ratio had 1.56- fold ( 95% confidence interval: 1.14 to 2.13; p 0.006) increased progression compared with those with the lowest level. Among participants without baseline plaque, TPA in re- examination was larger by 9.4 mm2 in the subgroup with the highest level when compared with the lowest level. CONCLUSIONS Our findings suggest that cholesterol- overloaded HDL particles are independently associated with the progression of carotid atherosclerosis. This may explain why in recent trials raising HDL cholesterol was not beneficial. This study strongly suggests that the combination of cholesterol content and particle number determines the antiatherogenic function of HDLs, rather than either parameter alone. ( J Am Coll Cardiol 2015; 65: 355- 63) c 2015 by the American College of Cardiology Foundation.