The normal content of eosinophils in the adult colon and the criteria for the histopathologic diagnosis of eosinophilic colitis remain undefined. This study aimed at: (1) establishing the numbers of eosinophils in the normal adult colon; and (2) proposing a clinicopathologic framework for the diagnosis of primary colonic eosinophilia and eosinophilic colitis. To accomplish these goals, we counted the eosinophils in the right, transverse, and left colon of 159 adults with normal colonic histology. Using a database of 1.2 million patients with colonic biopsies, we extracted all adults with a diagnosis of colonic eosinophilia. We reviewed the slides from all cases and captured demographic, clinical, and pathologic data, including information about eosinophilia in other organs. We then compared the clinical manifestations of the study patients (those with no identifiable cause of eosinophilia) to those of patients with other types of colitis. The normal eosinophil counts (per mm2) were 55.7±23.4 in the right, 41.0±18.6 in the transverse, and 28.6±17.2 in the left colon. Of the 194 study patients (eosinophil counts 166–5050/mm2), 63 were asymptomatic and had a normal colonoscopy. Diarrhea and abdominal pain were the commonest indications for colonoscopy (38% and 27%, respectively) among the 131 patients who had symptoms, endoscopic abnormalities, or both. Neither clinical manifestations nor endoscopic appearance were sufficiently characteristic to elicit the suspicion of colonic eosinophilia. In conclusion, primary colonic eosinophilia was extremely rare in this series (<1 in 6000 patients); one third of these patients were asymptomatic. Their clinical manifestations were not distinctive and could not have led clinicians to suspect this condition; one third of the patients were asymptomatic. We suggest that regularly reporting high colonic eosinophilia may result in increased opportunities for clinicopathologic studies that might lead to a better definition of this still elusive entity.