Polymorphism in a DNA sequence has been observed on the 3'-flanking region of the human apoprotein-A-1 gene. The frequency of the heterozygous state in a healthy control population (n = 73) is around 0.05. However, 12 (frequency 0.34) out of 35 subjects with hypertriglyceridaemia were found to have the polymorphic site, and 2 were homozygous for this variant. The mutant allele may constitute a linkage marker for some abnormality within the apoprotein-A-1 gene, affecting either expression of or some minor structural modification of the A-1 apoprotein, that may predispose to hypertriglyceridaemia.