As the Morphometric Reference Center of the WHO-ISFC PBDAY Project (Pathobiological Determinants of Atherosclerosis in Youth) we are studying lesion involvement of left hemiaortas of young people from different parts of the world. Gross specimens were stained and photographed at the Malmö reference Center (Dir. Prof. N.H. Sternby). Percentage of lipid lesions were evaluated and probability maps of lipid lesion distribution at the aortic surface were obtained by image processing. In all samples from Italy, Mexico and Sri Lanka, high probability of lipid lesion occurrence was found between the emergences of the intercostal arteries, next to the emergence of the celiac trunk, between the emergence of the superior and inferior lumbar arteries and between the emergence of the 1st and the 2nd mesenteric arteries. Our data reveal that a) lipid lesions are localized in low shear-stress regions or in turbuecent regions; b) our samples taken from groups of different geographical origin present the same lipid lesion distribution at the aortic surface.