The importance of the apolipoprotein A5 (APOA5) gene in determining plasma triglyceride (TG) levels has been demonstrated in transgenic and knockout mice and confirmed by human association studies in different ethnic groups.We screened for nonsynonymous APOA5 mutations in patients with plasma TG levels >10 mmol/L. The coding sequence and promoter region of the APOA5 gene were sequenced in 95 individuals with severe hypertriglyceridemia (HTG). A large population sample of 3,202 individuals was screened by PCR and restriction analysis for presence of detected mutation.In total, three heterozygous carriers of 944C>T (Ala315>Val) were identified in the severe HTG patients, while 22 carriers were identified in the population sample. The rare allele frequency of the Val315 was significantly higher in the HTG sample than in controls (0.016 vs. 0.003, pVal does not play any dominant/important role in the genetic determination of plasma TG levels, but the increased frequency in HTG patients compared to controls suggests that it might interact with other gene variants to cause HTG.