Fasting serum lipoproteins (LP) were separated into VLDL, LDL and HDL by ultracentrifugation and the content of cholesterol and triglycerides analyzed in each LP class in 69 consecutive men attending a lipid clinic. The effect of using different cut off points in the definition of HLP was discussed. It was pointed out that by using lower values for the cut off points there is only an increase in the number of subjects with hyperlipoproteinemia (HLP) but also shifts in the proportions between the various types of HLP, particularly with increase in the amount of types IIB and IV. In addition to the LP abnormalities inherent in the definition of each type of HLP other LP abnormalities were observed. Thus type IIA HLP had cholesterol rich VLDL with an increased ratio cholesterol/triglycerides. The types of HLP with increased VLDL triglycerides had characteristic changes in both LDL and HDL. For LDL its triglyceride content was increased and HDL showed a lowering of its cholesterol content and a rise in triglycerides. These changes were more pronounced the higher the VLDL triglyceride concentration was. Both normo- and hyperlipoproteinemic subjects having the second pre-beta LP on agarose gel electrophoresis of the VLDL fraction called LPB (Late Pre Beta) had two characteristic LP features. VLDL had an increased cholesterol/triglyceride ratio. LDL had a raised triglyceride content. The relation of LPB to so called intermediary particles was discussed. When the SPB (Sinking Pre Beta) LP variant was present this had negligible effects on the LP composition. Type IIA was present in 14 percent of the 609 men. None had tendinous xanthomata. The LP pattern of this common type IIA was compared to the LP pattern of the uncommon type IIA where tendinous xanthomata are present, called IIA-X. Type IIA-X had much higher LDL cholesterol and in relation to the triglyceride content a more cholesterol rich LDL. Furthermore type IIA-X had lower VLDL lipids than the common type IIA, but type IIA-X was more cholesterol rich. The differences in clinical appearance as well as in quantitative and qualitative LP composition between the common type IIA and type IIA-X makes it important to separate these two types of HLP from each other.