Peroral small intestinal enteroscopy was performed in 35 consecutive patients with gastrointestinal bleeding of unknown origin by using a prototypic Sonde-type enteroscope. The median value for patient age was 69 years; duration of bleeding, 2 years (range, 2 months to 9 years); and transfusion requirements, 9 units. Bleeding was occult in 54% of these patients. Placement and passage of the enteroscope was performed by a gastrointestinal endoscopy assistant. Mean passage time was 4.3 hours. Complete passage was obtained in 14% of the patients, passage into the mid-distal ileum in 69%, and passage only into the jejunum in 17%. The diagnostic yield was 26%, with the majority of lesions encountered being mucosal vascular malformations. Small intestinal enteroscopy is a reasonable diagnostic procedure before embarking on visceral angiography and surgical exploration when standard endoscopic and radiologic methods fail to disclose a diagnosis.