Sixty five of 70 consecutive patients with undiagnosed gastrointestinal blood loss were examined using the new technique of small bowel enteroscopy. Using a balloon driven sonde enteroscope (SIF-SW) extended views of the small bowel were obtained as far as the distal ileum. Medium length of small bowel examined was 140 cm (range (30-200 cm). All patients studied had a normal upper gastrointestinal endoscopy. Nineteen (41%) of 46 anaemic rheumatoid arthritis patients taking non-steroidal antiinflammatory drugs (NSAID) and three (27%) of 11 patients with unexplained iron deficiency, were found to have small bowel lesions to account for their anaemia. Small bowel lesions were found in a further three of eight (37%) patients with acute gastrointestinal bleeding. The procedure failed or was terminated in five patients. Small bowel enteroscopy has considerable potential in the investigation of undiagnosed gastrointestinal blood loss and deserves more widespread application.