Small dense low-density lipoproteins (LDLs) have been associated with coronary heart disease (CHD) and type 2 diabetes in previous studies. However, the significance of small dense LDLs as a coronary risk factor for subjects with type 2 diabetes remains unclear. We measured mean LDL particle diameter by gradient gel electrophoresis (Quantimetrix Lipoprinttrade mark LDL System) in 44 type 2 diabetes patients, 100 CHD patients, 35 CHD patients with type 2 diabetes and 88 age-matched control subjects. Mean LDL particle sizes were significantly smaller (p < 0.05, Mann-Whitney test) in diabetes mellitus-only patients (25.7 +/- 0.8 nm), CHD-only patients (25.2 +/- 1.4 nm), and CHD patients with diabetes mellitus (24.9 +/- 1.6 nm) than in controls (26.2 +/- 1.4 nm). As for the mean LDL particle size, the prevalence of small dense LDLs (mean diameter < or =25.5 nm) was higher (p < 0.05, chi(2) test) in all disease groups than in controls. Furthermore, the LDL particle size in CHD patients with diabetes mellitus (24.9 nm) was significantly smaller (p = 0.049) than in diabetes mellitus-only patients (25.7 nm). Multiple logistic regression analysis revealed that small dense LDL was independently associated with the incidence of CHD in all subjects [odds ratio (OR) 4.4, 95% confidence interval (CI) 1.2-16.1, and OR 4.5, 95% CI 1.2-17.3, p < 0.05]. In this study the LDL particle size did not further decrease in CHD patients even when they had diabetes. However, the presence of CHD affects the LDL particle size in diabetes patients. These results suggest that identifying LDL size as a routine laboratory test could be used as a marker for CHD risk in both diabetic and non-diabetic Korean populations.