In order to study the response of cholesteryl ester transfer activity (CETA) to alteration in diet in humans we carried out a longitudinal study in hyperlipidaemic patients. Five subjects, all hyperlipidaemic, were first given a low fat diet for 7 to 15 days and then a diet high in fat for 7 to 21 days. In four out of five patients both diets were low in energy. The plasma lipoprotein levels and CETA were measured daily. All patients responded on the low fat diet by a decrease in total cholesterol and total triglycerides. No response on the high fat diet was noticed except in the patient for whom the high fat diet was also high in energy. In all patients the changes in CETA ran parallel to changes in (VLDL + LDL) cholesterol. It is concluded that diet induced changes in (VLDL + LDL) cholesterol are accompanied by similar changes in CETA in hyperlipidaemic subjects.