Floxuridine (5-FUDR) is an antipyrimidine used in the treatment of metastatic colorectal carcinoma. Bowel toxicity, manifested predominantly by diarrhea and abdominal pain, is one of the main complications of 5-FUDR therapy. Six patients who had received 5-FUDR infusion either by hepatic arterial or systemic intravenous routes subsequently developed severe diarrhea and were evaluated by small-bowel series. Radiographic changes were largely confined to the ileum. In four of six patients, the entire ileum or its more distal part was markedly narrowed. In the other two patients, changes consisted of thickening or effacement of the mucosal folds in the distal ileum. The symptoms of all six patients resolved after discontinuation of 5-FUDR therapy. This was accompanied by improvement in the radiographic appearance of the ileum in three patients who underwent a subsequent small-bowel series. The finding of reversible ileal changes, particularly extensive or segmental narrowing, seems to be a characteristic radi...