A patient with mast cell disease of the small bowel is described in whom clinical, histological, and ultrastructural studies served to delineate the characteristic features of the disease. Urticaria pigmentosa, steatorrhea, eosinophilia, absence of antireticulin antibodies, and submucosal nodularity seen on radiographic study of the duodenum were the clinical characteristics. The endoscopic appearance was that of severe exudative duodenitis. The histology of the small intestinal mucosa showed crypt cell destruction and villous atrophy. Marked infiltration of the lamina propria with mast cells, eosinophils, and neutrophils was also distinctive. The enterocytes retained their columnar epithelium, confirmed on electron microscopy. The fine structural abnormalities of the mast cells are demonstrated for the first time. Degranulated mast cells predominated within the lamina propria and none was seen among the epithelial layers. The mast cell nuclei were irregular, often binuclear, and showed loss of their normal heterochromatin pattern. In their cytoplasm only few granulated bodies were seen and even more rarely inclusions with whorls and scrolls. We conclude that the clinical, histopathological, and ultrastructural appearances in mast cell disease of the small bowel are distinctive and should be used as criteria for diagnosis. Care should be taken in the evaluation of the number of mast cells since the demonstration of these cells may be affected by various fixing and staining techniques.