Possible errors in diagnosing rare sources of bleeding were analysed in 433 consecutive patients examined for upper gastrointestinal bleeding between May 1987 and June 1990. Rare lesions not necessarily recognizable as sources of bleeding were found in 14 patients (8 women and 6 men; mean age 66 [46-82] years). In 9 of the 14 the lesion was above the ligament of Treitz and thus accessible to routine esophagogastroduodenoscopy. Nonetheless, in 4 of these 9 cases the source of bleeding was not recognized at the first gastroscopy. They were all lesion in the region of the horizontal duodenum or Treitz's ligament (3 duodenal carcinomas, 1 aortoduodenal fistula). This finding indicates that, if the deep duodenum cannot be assessed with certainty in the initial gastroduodenoscopy, the region should be selectively studied endoscopically or radiologically before more invasive examinations are undertaken. In the remaining 5 of the 14 patients the source of bleeding was distal to Treitz's ligament. After the usual diagnostic tests (esophagogastroduodenoscopy, coloscopy, small intestine double-contrast imaging, angiography, scintigraphy) the sources of bleeding (angiodysplasias, jejunitis, enteritis) were confirmed in four patients by intraoperative ileo-jejunoscopy. In three of the latter cases bleeding did not recur after operative intervention, in one patient with multiple angiodysplasias severe bleeding recurred immediately after operation.