To examine how prevalence of the small dense LDL phenotype (LDL particle diameter < or =25.5 nm) is associated with coronary artery disease (CAD) in type 2 diabetic and non-diabetic Japanese men, an ethnic group with a low incidence of CAD, 85 non-diabetic men and 45 type 2 diabetic men with angiographically documented CAD, and 142 control men and 76 type 2 diabetic men without CAD were studied. Mean LDL particle diameter was determined using 2-16% polyacrylamide gel electrophoresis. LDL particle diameters in CAD patients were much smaller than those in controls (25.2+/-0.7 vs. 26.0+/-0.4 nm, mean+/-S.D., P<0.0001). LDL size was smaller in diabetic subjects (25.6+/-0.6 nm) and became even smaller in diabetics with CAD (25.0+/-1.0 nm). Prevalence of small dense LDL was markedly higher in both non-diabetic and diabetic CAD patients than that in non-diabetic and diabetic patients without CAD (71, 76, 23 and 42%, respectively). CAD patients had lower HDL-cholesterol and apo A1 levels, and higher triglyceride levels than those in diabetic and non-diabetic CAD-free patients, while total- and LDL-cholesterol levels were even lower in CAD group, and remnant-like particle-cholesterol, lipoprotein (a) and insulin levels were comparable among four groups. LDL size was significantly associated with triglyceride, HDL-cholesterol and glycemic control. Logistic regression analysis revealed that the small dense LDL phenotype was significantly associated with the incidence of CAD independent of low levels of HDL-cholesterol or high levels of triglyceride in both non-diabetic and diabetic cases. These results suggest that high prevalence of small dense LDL is a leading cause of CAD in both diabetic and non-diabetic Japanese men. Type 2 diabetes shows a greater capacity to reduce LDL size, which may contribute to the high incidence of CAD in the diabetic population.