The background, clinical signs, blood biochemistry and management of 18 cases of equine hyperlipaemia are described. Eleven of the animals were Shetland ponies, four were Welsh mountain ponies or their crosses, one was a fell pony and two were riding ponies of mixed breeding. Their average age was nine years. Fourteen of the cases were mares, of which nine were in foal and two were lactating; the remainder were geldings. Underlying or concurrent diseases were identified in only six animals, but in one other animal the hyperlipaemia appeared to have been precipitated by stress, and in another by undernutrition to prevent laminitis. Twelve of the animals were considered obese. There was no age, seasonal, or geographic bias to the distribution of cases. Plasma triglyceride concentrations were increased by between five- and 80-fold, and ranged from 4.7 to 78.8 mmol/litre. There was biochemical evidence of hepatic damage in 17 cases, of renal insufficiency in 15, and pancreatic pathology in three cases. Four animals were euthanased without therapy. The others were treated with oral glucose solutions, which were supplemented with injections of insulin and heparin in four cases, and insulin alone in two cases. Eight of the treated animals died, to give an overall mortality of 67 per cent. The outcome of the treatment was unrelated to the degree of hypertriglyceridaemia, to the presence and severity of hepatic, renal or pancreatic pathology or to the therapeutic regimen.