Familial combined hyperlipidaemia (FCHL) is a common, multifactorial disorder associated with elevated levels of plasma triglyceride, cholesterol, or both^1–3. A characteristic feature is increased secretion of very low density lipoproteins (VLDL) and apolipoprotein B (apoB; refs 3,4). Although FCHL is the most common cause of premature coronary artery disease (CAD), accounting for over 10% of cases, its aetiology remains largely unknown^3–6. One powerful approach to the dissection of complex genetic traits involves the use of animal models⁷. We have identified a mouse strain, HcB-19/Dem (HcB-19), which exhibits hypertriglyceridaemia, hypercholesterolaemia and elevated levels of plasma apoB. Like FCHL patients, HcB-19 mice also exhibit increased secretion of triglyceride-rich lipoproteins, and their hyperlipidaemia becomes progressively more severe with age. It is likely that the hyperlipidaemia results from a mutation of a novel gene that arose during development of strain HcB-19. We mapped the hyperlipidaemia gene (Hyplip 1) to the distal portion of mouse chromosome 3. This region is syntenic to human chromosome 1q21–q23, which has recently been shown to harbour a gene associated with FCHL in families from a Finnish isolate (see accompanying manuscript by Pajukanta et al., ref. 8).