Intestinal mucosal biopsy immunoglobulin content in a patient with eosinophilia, allergic gastroenteropathy (atopy, food sensitivities, protein-losing enteropathy, iron deficiency anemia, and growth retardation), and hyperimmunoglobulinemia E (68,000 units) was compared with that of a control group and a group with eosinophilic gastroenteritis who had no atopic features. The patient had no evidence of cellular or humoral immunodeficiency but was severely hypersensitive to multiple inhalant and dietary allergens. In contrast to the findings in patients with nonallergic eosinophilic gastroenteritis, whose intestinal immunoglobulin content was similar to that of controls, tissue immunoglobulin IgE and IgG was markedly increased in the allergic patient. These results support an intestinal reaginic mechanism in the etiology of the allergic form of eosinophilic gastroenteritis.