It is generally accepted that platelet half-life is shortened in atherosclerotic vascular disease. Concerning changes due to hyperlipoproteinemia (HLP), however, there exist only few data. Therefore, we examined the platelet-half life in 60 patients with recently discovered HLP type IIa, IIb and IV according to Fredrickson before treatment in comparison to 60 controls. 33 of the HLP-patients had no clinical symptoms of angiopathy, 27 patients suffered from peripheral vascular disease or from coronary heart disease as verified by angiography. The labelling of autologous platelets was performed with 100 microCi of 111Indium-oxine-sulfate at 37 degrees C for 5 minutes. The mean labelling efficiency was 90%, the recovery after 2 hours about 70%. Serum lipoproteins were estimated by means of ultracentrifugation and polyanionprecipitation according to Lipid Research Clinic Methods. In the patients with HLP platelet half-life was significantly shortened in comparison to the control group (p less than 0.01). These changes were most pronounced in patients with HLP-type IIa and with atherosclerotic lesions, respectively. In patients with HLP-type IIa a very close correlation could be demonstrated between platelet half-life and LDL-cholesterol (r = -0.72; p less than 0.001) as well as total cholesterol (r = -0.73; p less than 0.001). These data prove that in HLP in-vivo platelet function as measured by platelet survival is significantly influenced even before the occurrence of clinically relevant symptoms of atherosclerosis.