Forty-five patients with proved gastrointestinal amyloidosis were examined to study the clinical and pathological features and to determine the correlation with amyloid fibril proteins. The examinations included physical examination, laboratory study, plain X-ray film of the abdomen, gastrointestinal radiography, gastrointestinal endoscopy, endoscopic biopsy, surgery, and autopsy. The results were as follows: 1) Amyloid fibril proteins consisted of amyloid light chain protein (AL) in 7 patients, amyloid A protein (AA) in 36, beta 2-microglobulin (AH) in one, and prealbumin (AF) in one. 2) At the time of examination, gastrointestinal symptoms were present in 43 (96%) of the 45 patients. The incidence of diarrhea and gastrointestinal bleeding was higher in patients with the AA type than in those with other types, whereas abdominal distension, nausea, and vomiting were more frequently evident in patients with the AL type. 3) The incidence of hypoalbuminemia, hypokalemia, and positive occult blood was higher in patients with the AA type, whereas abnormal electrocardiogram and cardiac failure were more frequently observed in patients with the AL type. The thyroid gland was enlarged in 7 patients with the AA type, and the macroglossia was evident in only 2 patients with the AL type. 4) The frequency of radiographic abnormalities in the gastrointestinal tract was as follows: 79% of the patients in the jejunum and ileum, 61% in the duodenum, 53% in the stomach, 32% in the colorectum, and 14% in the esophagus. Fine granular shadows were most frequently seen in patients with the AA type, whereas multiple polypoid protrusions and thickening of the folds were characteristic in patients with the AL type. 5) Endoscopic abnormalities were found as follows: 82% in the duodenum, 81% in the jejunum, 73% in the stomach, 42% in the colorectum, and 20% in the esophagus. Endoscopy revealed fine granular appearance in 31 (86%) of the 36 patients with the AA type, whereas multiple polypoid protrusions and thickening of the folds were evident in 5 (71%) of the 7 patients with the AL type. 6) On the histological examination of the biopsy specimens, amyloid deposition was found in the duodenum in 98% of the 45 patients studied, in the jejunum in 96% of the 26 patients, in the antrum in 93% of the 41 patients, and in the rectum in 86% of the 43 patients. 7) Histological findings of the biopsy and autopsy specimens revealed that the the degree of amyloid deposition was the highest in the duodenum and jejunum of the gastrointestinal tract.(ABSTRACT TRUNCATED AT 400 WORDS)