Histological, histochemical and electron-microscopical examinations have been performed on xanthoma of the skin and palmar creases in 5 patients with hyperlipoproteinemia type III, to follow the dynamics of xanthoma formation. The results were as follows: 1. There are no basic differences in histomorphology between xanthomas in type III-HLP and other types of HLP. Typical findings are: Rather dense distribution of relatively small foam cells within the dermis, only a few foam giant cells, great amounts of cholesterol intra- and extracellulary, and circumscribed necrobioses. 2. Xanthoma formation in type III-HLP is induced by pathological plasma lipoproteins. These seem to pass the endothelial barrier via pinocytosis or/and filtration. Between endothelial cells and their basal lamina plasma components sometimes could be observed. 3. The pathological lipoproteins induce a phagocytic reaction with foam cell formation in the dermal tissue. At first an augmentation of lymphocytoid or histiocytoid tissue macrophages could be detected in the dermis and especially in the perivascular region of venous blood vessels in the middermis. Perithelial cells as well as lymphocytoid and histiocytoid tissue macrophages engulve lipoproteins. Cytoplasmic enrichment of vacuoles with an electron opaque content (lipoproteins) are typical for type III-HLP; they lead to formation of typical foam cells. Within those cells lysosomal intracellular degradation of the engulved lipoproteins takes place; free cholesterol cristals, phospholipid-containing myelinic figures and residual bodies are endproducts of this process. They can lead to cellular necrosis with cellular remnants, lipoprotein vacuoles, free cholesterol and phospholipids lying free within the dermal tissue. 4. Factors are not well understood which control circumscribed xanthoma formation in type III-HLP. 5. These investigations show, that xanthomas are reactive new formations. The leading cellular substrate is the tissue macrophage, which is transformed into a foam cell by intake of lipoproteins. Foam cell formation, however, is not the result of a simple process of storage: after lipoprotein intake numerous, to a part lysosomal processes of chemical degradation and transformation, take place with an appearance of intracellular endproducts (free cholesterol, residual bodies, phospholipid-containing membranes). It is suggested that the dynamics of xanthoma formation might be importance in understanding of the process of atherosclerosis in those patients.