The effect of isopropyl-[4'-(p-chlorobenzoyl)-2-phenoxy-2-methyl]-propionate (LF 178; procetofene; Lipanthyl), a new hypolipidemic agent, has been evaluated in 191 patients affected by endogenous hyperlipoproteinemia (IIa, IIb and IV). Data were collected from patients on active drug trial from 1 to 18 months. The daily administered dose varied from 200 to 400 mg according to pretreatment dietary stabilized lipid values. Under the controlled experimental conditions described, LF-178 lowered cholesterol by 20 to 36% in types IIa and IIb and triglycerides by 30 to 50% in types IIa and IV of H. L. P. A comparative trial of LF 178 and clofibrate indicates that the drug is significantly more potent than the reference compound after 1 month of treatment. Tolerance has always been excellent throughout the study.