The ultrastructural changes in the jejunal mucosa of 11 male patients, three with clinical AIDS, five with AIDS related complex-progressive generalized lymphadenopathy (ARC-PGL), and three who were only HIV antibody positive, were studied. In the enterocytes, major abnormalities were proliferation of smooth endoplasmic reticulum mitochondrial changes, vacuolization of cells, and fat hold up. In the lamina propria, degeneration of enteric nerve axons and smooth muscle were seen. Microvasculature showed both endothelial cell degeneration and hyperplasia. The presence of tubuloreticular inclusions in endothelial cells paralleled the stage of the disease. Since none of the 11 patients had any opportunistic infection, these changes are likely to be the effect of HIV infection.