Evaluation of the mucosal eosinophil content is important in the interpretation of endoscopic biopsies, as high eosinophil densities might reflect allergic gastrointestinal disease. Reference ranges gleaned from the literature have an upper limit of normal varying from 6 to 20 eosinophils per 400 x high power field. Preliminary data suggest a geographic variation with higher eosinophil counts in the southern United States. We examined intestinal tract mucosa from 44 infants and children who died suddenly and unexpectedly in northeastern Texas. Subjects ranged in age from 3 wks to 17 yrs and were without known gastrointestinal disease. Using formalin-fixed, hematoxylin and eosin-stained 4-microns sections, intramucosal eosinophils were counted in ten consecutive high power fields and the mean eosinophil count determined. Twenty-three subjects (52%) had counts of > 20 eosinophils/high power field from at least one site. There was no correlation with age, sex, season, or cause of death. In a subset of 11 subjects, more extensive sampling showed the cecum and appendix to have the highest concentration of eosinophils and relatively low counts in the stomach and distal large intestine. These observations correlate with our impression that increased numbers of eosinophils, particularly in the proximal colon, are a common feature of otherwise unremarkable pediatric endoscopic biopsies. Efforts to distinguish the proportion of activated eosinophils in B5-fixed mucosal biopsies using the EG2 monoclonal antibody were unsuccessful, as this antibody does not appear specific for activation in B5-fixed tissue. Mucosal eosinophil counts should be interpreted with caution in geographic areas where a high background count is endemic.