Background The apolipoprotein A5 gene (ApoA5) plays an important role in modulating triglyceride metabolism. Polymorphisms of ApoA5, including -1131T>C and c.553G>T (G185C), have been reported to correlate with hypertriglyceridemia (HTG). In the present study the relationships of 5 single nucleotide polymorphisms, including the -1131T>C, c.56C>G, IVS3+476G>A, c.553G>T, and c.1259T>C polymorphisms of ApoA5, with HTG were investigated. Methods and Results The study group comprised 95 Japanese patients with HTG and 119 unrelated normolipidemic subjects. Frequencies of the C allele of -1131T>C (0.511) and the T allele of c.553G>T (0.205) in the hypertriglyceridemic patients were significantly higher than in the normolipidemic subjects (0.315 and 0.105, respectively). The c.56C>G (S19W) polymorphism was not observed, and the other 4 polymorphic sites were in strong linkage disequilibrium. Five of the 8 detected haplotypes with the C allele of -1131T>C correlated with HTG. Promoter activities of ApoA5, including that with the -1131T>C polymorphism, were estimated using a luciferase assay. Analysis of ApoA5 promoters showed that the -1131T>C polymorphism alone had no effect. Comparison of expression of mutant G185C and wild-type ApoA5-green fluorescent protein (GFP) in HepG2 cells showed that ApoA5-GFP was abundant in punctate endosome-like structures, and ApoA5 (G185C)-GFP expression resembled that of the wild type. Conclusions The -1131T>C and c.553G>T (G185C) polymorphisms correlated with HTG in this Japanese population, but neither polymorphism directly affected ApoA5 expression. (Circ J 2007; 71: 746 - 752)