The frequency of the opportunistic infections of the duodenum in AIDS patients was determined by way of histologic study in 207 patients between January 1987 and June 1991. All cases had serial paraffin sections, run through HES, PAS, Giemsa, Brown-Brenn, and Zieh-Neelsen stains, and 20 cases had in addition cytologic and electron microscopic study. 63 patients had opportunistic infections (10 cryptosporidiosis and 2 isosporiasis; 12 mycobacterial enteritis; 15 CMV enteritis; 7 candidosis; 7 intestinal microsporidiosis confirmed by electron microscopic examination; 12 Giardiasis; 3 duodenal leishmaniasis; 1 intestinal cryptococcosis). Multiple concurrent infections were noted in 6 cases. A mild to severe villous atrophy was observed in 28 cases, associated with opportunistic infection. A patchy distribution of pathogen agent was noted in 34 cases, and 37 cases were associated with oesophagal candidosis. This study points out the value of histologic examination of intestinal biopsy for the diagnosis of systemic infections as well as of unusual parasitosis, and the necessity for multiple endoscopic biopsies because of the frequent patchy distribution of pathogens.