A DNA sequence polymorphism, revealed by digestion of human DNA with the restriction endonuclease Sst-1 and hybridization with an apolipoprotein A-I complementary DNA clone, has been shown to be located in or close to the 3' noncoding region of the apolipoprotein C-III gene. This polymorphism is found in significantly increased prevalence (P less than 0.001) in Caucasian hypertriglyceridemic subjects compared with race-matched controls, and its distribution in normal individuals of differing racial origins is reported. Furthermore, no alteration of high density lipoprotein or apolipoprotein A-I and apolipoprotein C-III phenotypes was observed in individuals with or without the polymorphism.