The cytologic findings of duodenal smears in diarrheic HIV-positive patients were compared with results of histologic and ultrastructural studies. This study included 50 diarrheic patients undergoing upper gastrointestinal endoscopy. Duodenal biopsies were taken with touch preparations. Smears were stained with Giemsa and then with Ziehl-Neelsen or the PAS method. The biopsy specimens were then processed in a standard fashion for histologic and electron microscopy examination. Cytologic findings included 8 cases of intestinal microsporidiosis, 4 cases of cryptosporidiosis, and 3 cases of atypical mycobacteriosis. Histologic and ultrastructural studies confirmed these diagnoses and showed 8 cases of CMV enteritis, of which 4 were associated with other enteric pathogens. Duodenal cytology complements endoscopic biopsy in the work-up of diarrhea in AIDS patients and may provide a rapid diagnosis especially in the case of intestinal protozoonosis or atypical intestinal mycobacteriosis.