Small dense low-density lipoprotein (sd LDL) is more atherogenic than large buoyant (lb) LDL, which is relatively high in particle number (as estimated by apolipoprotein [apo] B) and poor in cholesterol (C) content. Because recent epidemiological studies have shown that serum apo B is a stronger predictor of the risk of coronary heart disease (CHD) than LDL-C, we assumed that this strong predictive power of apo B for CHD is attributable to a close association with sd LDL concentration. On the other hand, non-HDL-C has been proposed as an integrated CHD risk marker containing all atherogenic apo B-containing lipoproteins. We examined which apo B or non-HDL-C is superior marker in reflecting sd-LDL particles. Eighty-one subjects with dyslipidemia were studied. Sd LDL (density, 1.044 approximately .063g/ml) and large buoyant LDL (density, 1.019 approximately 1.044g/ml) were separated by the ultracentrifugation method and LDL size was measured by gradient gel electrophoresis. LDL-C, non-HDL-C, and apo B were highly associated with each other(r=0.78 approximately 0.87), and all were associated with C, apo B, and TG in sd-LDL. However, multivariate regression analysis revealed that only apo B was constantly and independently associated with sd-LDL concentrations. Mean LDL diameter was negatively associated with apo B but not with non-HDL-C. These results suggest that apo B is superior to non-HDL-C in reflecting a potent atherogenic lipoprotein, sd-LDL, which may explain a potent predictive power of apo B for CHD.