Background: Epidemiological studies suggest that atherogenic small, dense low-density lipoprotein is an independent risk factor for atherosclerosis. It has been known that triglycerides/high-density lipoprotein cholesterol (TG/HDL-C) ratio represents the atherogenic lipoprotein and is one of the strongest predictors for ischemic heart disease. We investigated whether the T-1131C polymorphism in the apoprotein A5 (APOA5) contributes to the changes of TG/HDL-C and is affected by other factors such as age, body mass index (BMI), insulin resistance (IR), and hypertension status. Methods: Normal subjects (M:F=197:302) and hypertensives (M:F=320:372) were recruited. A T-1131C polymorphism in the promoter region of APOA5 was genotyped by SNP-IT assay. Results: The TG/HDL-C ratio in the subjects with C carrier was significantly higher than that of TT homozygotes both in normotensives and hypertensives. Multiple linear regression analysis indicated that TG/HDL-C ratio was significantly associated with APOA5 genotype (p<0.0001), IR (p<0.0001), BMI (p<0.0001), and total cholesterol (p=0.0216) in males, while it was associated with hypertension status (p<0.0001), APOA5 genotype (p=0.0005), BMI (p<0.0001), IR (p=0.0474), age (p=0.0154), and total cholesterol (p<0.0001) in females. Significant linear relationship was observed between TG/HDL-C ratio and BMI in normotensives, and between TG/HDL-C ratio and IR both in normotensives and hypertensives. In univariate analysis to assess the influence of APOA5 genotype on the relationship between TG/HDL-C ratio and BMI, the slope was shifted upward both in male and females and presented a steeper slope (p <0.001) in C allele carriers than in TT homozygotes in males. Univariate analysis on relationship between TG/HDL-C and IR showed that the slope was shifted upward both in male and females (p<0.001). Conclusion: Our results indicated that T-1131C polymorphism in the APOA5 gene affects on TG/HDL-C ratio, and higher TG/HDL-C ratio was noted in C carriers at any BMI or insulin sensitivity when it compared to that of TT homozygotes. However, the linear relationship between TH/HDL-C and BMI or IR suggest that reduction of BMI and IR can reduce the TG/HDL-C ratio in any genotype.