Combined treatment of hyperlipidaemia with a 1000-calorie-diet and d-triiodothyronine resulted in a lowering of serum cholesterol, triglycerids and lipoproteins. Concerning cholesterol and beta-lipoproteins there was an addition of reducing effects of simultaneous dietary and drug treatment. Triglycerids are influenced by the combined treatment or by dietary treatment alone to the same extent. The levels of free fatty acids rise during combined treatment while there is no change by diet alone. These observations indicate a most successful lowering of cholesterol and beta-lipoproteins by the combined therapy, but there is an increase of free fatty acids as a metabolic side effect of d-triiodothyronine.