Cell-to-cell interactions are mainly involved in the control of the proliferation, migration, differentiation and function of different cell types in a wide range of tissues. In the arterial vessel wall, human arterial endothelial cells (haEC) and smooth muscle cells (haSMC) coexist in close contact with each other. In atherogenesis, haSMC can migrate from the media to the subintimal space to form fibromuscular and atheromatous plaques. In the present study, a transfilter coculture system is described, in which the interface between haSMC and confluent or proliferative haEC can be studied in detail. Cells were cocultured on the opposite sides of a porous filter which separates both cell types like the internal elastic lamina in vivo. In cocultures containing proliferative haEC, haSMC growth was significantly stimulated (33.4 ± 5.7 cells/section, p < 0.05) compared to haSMC monocultures (22.9 ± 2.5 cells/section) and cocultures containing confluent haEC (15.6 ± 2.9 cells/section). If confluent haEC were injured mechanically, haSMC growth increased highly significantly (71.3 ± 16.8 cells/section, p < 0.001). Thus, cell-rich proliferates containing 5-7 layers of haSMC embedded in extracellular matrix were formed after 14 days. On the other hand, after haSMC migration to the endothelial side had occurred, the addition of LDL and monocytes to cocultures with arterial media explants and haEC resulted in the formation of lipid-rich, low-cellular structures. After 28 days, characteristic in vitro plaque growth was induced; the plaque contained a lipid core with predominantly necrotic cells, extracellular lipid accumulations, atypically shaped lipid-loaded haSMC and macrophages, similar to in vivo foam cells, as well as an increased amount of extracellular matrix (collagen I, III and IV). These areas were surrounded by typical fibromuscular caps consisting of smooth muscle α-actin-positive haSMC. Finally, the formation of capillaries by haEC could also be observed within these structures.