Plasma lipid and lipoprotein pattern as well as platelet aggregation and [14C]serotonin release were studied in five male patients with primary Type V hyperlipoproteinemia and in age-matched healthy control subjects. Plasma very-low-density lipoprotein (VLDL) cholesterol, triglycerides (TG) and protein were significantly elevated in the patients, and this resulted in 54% and 32% increments in cholesterol:protein and TG:protein ratios, respectively. In both low-and high-density lipoproteins (LDL and HDL), the cholesterol:protein ratio was normal, but the TG:protein ratio increased threefold. In the patient VLDL, apolipoprotein (apo) E and the sialyzed apo C-III (C-III2) were elevated, but apo C-III0 was decreased; whereas in the patient HDL, sialyzed apo C-IIIs were lower than in the normal HDL. Platelet function was found to be decreased in our patients in comparison with the normal group. Upon incubation of plasma lipoproteins derived from both groups with normal washed platelets, all patient plasma lipoproteins caused decreased platelet function in comparison with the normal lipoproteins. This study indicates plasma lipoprotein abnormalities in Type V hyperlipoproteinemia and may suggest that depressed platelet aggregation and release in these patients are a result of this normal lipoprotein pattern.