The first clinical thermofiltration procedures (plasma filtration near or above normal physiologic temperature) were performed on a secondary hypercholesterolemia patient. The intent of this filtration scheme is the selective removal of low density lipoproteins (LDL) and maintenance of high density lipoproteins (HDL) and other essential plasma proteins including albumin. The EVAL 4A filter, at a filtration temperature of 37.degree. C, was selected for on-line thermofiltration. The sieving coefficients of LDL + very low density lipoproteins (VLDL) cholesterol, HDL cholesterol, and albumin were 0.02, 0.75, and 0.78, respectively. These results are comparable to in vitro filtration tests using the plasma of the same patient. This system is simple to apply and does not require replacement solution. Compared with plasma exchange, a high return of HDL and other essential plasma solutes can be returned to the patient by this method.