Background : Severe hypertriglyceridemia is associated with an increase risk for acute pancreatitis. Mutations in the genes encoding lipoprotein lipase and apolipoprotein C2 are the well known genetic causes of severe hypertriglyceridemia. APOA5, encoding apo A-5, is a newly discovered gene implicated in triglyceride metabolism. Recently, apolipoproteinA5 was identified as a genetic cause of severe hypertriglyceridemia in several families. However, genetic variations of APOA5 and its association with triglyceride level in Thai patients are unknown. Objective : To determine the genetic variation of the APOA5 gene in Thai patients with very high levels of triglyceride Methods : The coding region and exon-intron junctions of the APOA5 gene were examined by sequencing in 50 Thai subjects with severe hypertriglyceridemia (triglyceride levels more than 10 mmol/L [886 mg/dl] on ≥ 2 occasions) and 59 controls. Results : We found several known variants in the APOA5 gene. Two variants, c.-3A>G and c.553G>T were found at significantly higher frequencies in the hypertriglyceridemia group compared with those in the control group (82% vs 31%, p < 0.001 and 30% vs 0%, p <0.001, respectively) Conclusion : The APOA5 c.-3A>G and c.553G>T variants may contribute to severe hypertriglyceridemia in Thai subjects.