Genome-wide association studies (GWAS) have identified multiple loci associated with plasma lipid concentrations^,,,,. Common variants at these loci together explain <10% of variation in each lipid trait^,. Rare variants with large individual effects may also contribute to the heritability of lipid traits^,; however, the extent to which rare variants affect lipid phenotypes remains to be determined. Here we show an accumulation of rare variants, or a mutation skew, in GWAS-identified genes in individuals with hypertriglyceridemia (HTG). Through GWAS, we identified common variants in APOA5, GCKR, LPL and APOB associated with HTG. Resequencing of these genes revealed a significant burden of 154 rare missense or nonsense variants in 438 individuals with HTG, compared to 53 variants in 327 controls (P = 6.2 × 10⁻⁸), corresponding to a carrier frequency of 28.1% of affected individuals and 15.3% of controls (P = 2.6 × 10⁻⁵). Considering rare variants in these genes incrementally increased the proportion of genetic variation contributing to HTG.