Four normal and two individuals with type IIa hyperlipoproteinemia were placed on the National Heart and Lung Institute Type IIa Diet (less than 300 mg of cholesterol per day, high polyunsaturated, low saturated fat diet) for 1 week and on a normal diet the following week. Plasma samples were obtained and the cholesterol contents of plasma and of very low density, low density and high density lipoproteins determined. The cholesteryl esters in one type IIa and two normal individuals were identified. The cholesteryl esters in type IIa very low density lipoproteins from blood drawn 45 min after the last meal in each dietary period, contained less 18:2 than from the normal. After the first dietary period, the very low density lipoprotein cholesteryl ester 18:2 content for the type IIa was 37.2M% and for the normals, 54.7M%. After the second dietary period, the corresponding values were 49.7M% and 56.7M%. Fasting samples had lower 18:2 contents in the low density lipoproteins from the Type IIa subject following both dietary periods and in the high density lipoproteins following the Type II diet.