Many subjects with relatively normal, or even optimal, levels of low-density lipoprotein cholesterol (LDL-C) have increased atherogenic lipoprotein particle concentrations (apolipoprotein B [apoB] and LDL particle number [LDL-P] determined by nuclear magnetic resonance spectroscopy [NMR]). Numerous analyses have demonstrated that apoB and LDL-P predict the risk of future cardiovascular events more robustly than LDL-C, as estimated using the Friedewald equation. Little is known about the relationship between an individual’s apoB and LDL-P level, and whether the relationship is comparable at different levels of LDL-C. The aim of this study was to examine the correlation between plasma apoB and LDL-P levels and specifically to evaluate the heterogeneity of LDL-P at low levels of apoB (, the 20th population percentile [78 mg/dL]). Data were derived from a group of consecutive patients added to a large, single laboratory database (LipoScience, Inc.) during a 1-week period in which a standard lipid profile, apoB, and LDL-P levels were available. When risk categories were assigned to the subjects using the Framingham Offspring Study’s population percentiles for apoB and LDL-P, there was good agreement between the two measures when LDL-C levels were high ($160 mg/dL). However, among individuals with low LDL-C and apoB, NMR analysis could identify a subgroup of individuals