The histopathological data presented support a new concept for the origin of the cells which cause intimal thickening of arteries. Arterial segments, isolated between ligatures, when examined by intravascular contrast techniques, showed penetration of vasa-vasora and formation of intra-arterial granulation tissue which produced myointimal thickening. Transitional forms between pericytes and myointimal cells were found. In autoradiographic studies on the incorporation of 3H-thymidine, DNA synthesis was first seen in the adventitia, fundamentally in the vasa-vasora pericytes, and in the adjacent media, and later in the intimal thickening. In arterial segments between ligatures typical intimal thickening was produced when intra-arterial granulation tissue was formed and the ligatures were removed thus restoring the circulation. These results were not produced when the arterial segment was sectioned lengthways between ligatures. It is suggested that this intimal thickening originates in cells from the vasa-vasora, in particular from pericytes.