Biopsy specimens of the gastric antrum, duodenum, and rectum from three patients with secondary amyloidosis were examined by electron microscopy in an attempt to determine the ultrastructural distribution of amyloid filaments and to identify any secondary changes in the covering mucosal epithelial cells. The characteristic amyloid filaments were seen in the walls of submucosal arterioles and mucosal capillaries deposited within the basal lamina surrounding the endothelial cells. Filaments were also sometimes seen within the muscularis mucosa. the overlying gastric and rectal epithelial cells appeared normal, but numerous curved bacilli were seen in close contact with the microvilli of the surface epithelial gastric cells. Duodenal columnar absorptive cells were vacuolated and contained prominent lysosomes. These changes are probably degenerative and may explain, at least in part, the development of malabsorption in some patients with intestinal amyloidosis.