The frequency of polymorphisms of the apolipoprotein (apo) B and E gene loci were studied in 59 patients with primary combined hyperlipidaemia, including 26 with familial combined hyperlipidaemia. Variation at the apo B gene locus was examined using restriction enzymes XbaI, EcoRI, and PvuII. At the apo E gene locus, variation causing the E2, E3, and E4 phenotypes was detected using the polymerase chain reaction in conjunction with allele-specific oligonucleotide probes. The frequency of apo B RFLPs in patients were not significantly different from healthy controls, however, the apo E4 allele was over-represented. This suggests that possession of the apo E4 allele may contribute to the development of the combined hyperlipidaemic phenotype and to familial combined hyperlipidaemia.