In 53 patients with chronic diarrhea ileoscopy was done following colonoscopy. Beside the microscopic examination, terminal ileum biopsies and mucosal smears were also performed. Endoscopy of the terminal ileum was abnormal in eight patients (15.1%); biopsy itself was diagnostic in 22 patients (41.5%): primary bile acid malabsorption with mucosal atrophy and reduced retention of 75ScHCAT (10), mucosal atrophy after cholecystectomy (4), Crohn's disease (6), backwash ileitis in ulcerative colitis (1), and postirradiation ileitis (1). Biopsies were normal but mucosal smear indicated the cause of diarrhea in a further 10 patients: giardiasis was found in 7, and candidiasis in 3 patients. All in all, endoscopy, biopsy and mucosal smear of terminal ileum showed a sensitivity of 58.5%. In 38 patients in whom laboratory, roentgenologic and endoscopic investigation failed to establish the etiology of diarrhea, the sensitivity of ileoscopy itself was 0%, of ileoscopy with biopsy 36.8% and ileoscopy with biopsy and mucosal smear 47.4%. We conclude that endoscopy, biopsy and mucosal smear of the terminal ileum are indicated in the investigation of patients with chronic diarrhea.