ABSTRACTThe term hyperlipidaemia is used to describe raised plasma concentrations of cholesterol and, or, triglycerides. These aqueous insoluble lipids are transported through plasma in special particles called lipoproteins of which there are four main types; chylomicrons, very low density lipoproteins (VLDL), low density lipoproteins (LDL) and high density lipoproteins (HDL). A transient rise in plasma triglycerides occurs following a meal as dietary fat is carried from the small intestine into the circulation by chylomicrons; this is called post prandial hyperlipidaemia. In addition, hyperlipidaemia is caused by defects in the metabolism of one or more of the lipoprotein classes which may be either genetic in origin or, more commonly in the dog and the cat, secondary to diabetes mellitus, hypothyroidism, hyperadrenocorticism, and renal and hepatic disease. Hypertriglyceridaemia is caused by reduced clearance of chylomicrons and VLDL, sometimes with overproduction of VLDL, whereas hypercholesterolaemia results from altered metabolism of LDL and HDL. Raised plasma triglycerides interfere with a number of clinical chemistry tests and may be associated with cutaneous xanthomata, vomiting and diarrhoea, peripheral nerve paralyses, seizures, pancreatitis, hepatosplenomegaly and lipaemia retinalis. The clinical manifestations of hypercholesterolaemia in the dog are few and largely confined to the eye. Diagnostic efforts should concentrate on determining whether the hyperlipidaemia is either genetic in origin or secondary to endocrine and systemic diseases. Plasma lipid concentrations usually return to normal with effective therapy of any underlying disease. Where no such disease can be identified, the hyperlipidaemia should be considered idiopathic in origin and the patient placed on a low fat diet.