Pirifibrate, a new hypolipemic substance, was studied in a multicenter trial for three months in 151 patients diagnosed as having type IIa, IIb or IV hyperlipoproteinemia (HLP). The patients admitted into the study had not received any hypolipemic medication for four weeks immediately before the study. We observed falls of between 24.3% and 20.0% in the average levels of plasma cholesterol in types IIa and IIb HLP. The triglycerides showed a decrease in average values, which varied between 31.0% and 38.6% in types IIb and IV HLP. The alpha-lipoproteins always increased in three types of HLP studied, while the pre-beta-lipoproteins showed a fall in averages in types IIb and IV HLP. The beta-lipoprotein levels were reduced in type IIa HLP and reached statistical significance.