The ultrastructure was studied of an umbilical artery from a newborn child delivered by a healthy mother who smoked as many as 80 cigarettes per day. Focal areas in the luminal part of the media appeared acellular consisting of collagen fibers only. In the deeper parts of the media the smooth muscle cells were hypertrophic and showed sign of proliferation. These smooth muscle cells had accumulated huge amounts of glycogen in the perinuclear region. The glycogen accumulations were always found in close connection with the mitochondria and often associated with lipid droplets. Occasionally phagocytosis and degradation of collagen by the smooth muscle cells were visualized.