A girl with the typical features of progeria died at the age of 10 years from cardiac failure following repeated myocardial infarctions. Serial studies of the serum lipids were made from the age of 2 years 3 months. Variable hyperlipidaemia, increased β- and pre-β-lipoproteins, and impaired clearing of absorbed dietary fat were shown. A polyunsaturated fat diet, which was started at the age of 2 years 5 months and continued throughout the rest of her life, controlled the hyperlipidaemia and the clearing of fat became normal. Pre-β-lipoprotein could still, however, be seen occasionally. The dietary regimen did not delay the development of atherosclerosis, the pathogenesis of which, in progeria, may be due to factors other than hyperlipidaemia.