Angiogenesis was examined by a three-dimensional model in vitro, using human umbilical vein endothelial cells (HUVECs) cultured in a collagen gel. An abundant capillary-like network with a lumen structure was identified histologically and shown to have formed at a collagen density of 0.05% or 0.10% instead of 0.15%, for either type I or type III collagen. At the same density, type III collagen induced a capillary-like network with HUVECs at an earlier stage of culture than type I collagen. In a two-dimensional culture, HUVECs were viable and proliferated to a great extent on the culture dish coated with collagen. This was particularly so in the case of type III collagen at lower density (5 ng/cm2). Type III collagen at a sufficiently low density is thus shown useful for studying angiogenesis in vitro. The capillary-like network that formed in the three-dimensional culture appeared somewhat labile, but became stable with the continuous addition of endothelial cell growth supplement and increase in the plating number of HUVECs.