Two siblings with eosinophilic gastroenteritis who presented with severe iron deficiency anemia and hypoalbuminemia associated with varying degrees of mucosal damage are described. Using a monoclonal antibody to the secreted form of eosinophil cationic protein, we demonstrated activated degranulating eosinophils in the gastrointestinal mucosa that correlated with the degree of histologic damage. This finding suggests that eosinophils may have a primary role in the tissue damage in this condition. Oral disodium cromoglycate (Nalcrom) failed to alter the clinical, radiologic, and histologic abnormalities. Prednisolone, however, was beneficial in 1 case but did not completely reverse the abnormalities.