Several systemic disorders and gastrointestinal diseases may be associated with increased colonic mucosal eosinophils, which may vary in number throughout the normal colon. Some investigators have proposed that colonic eosinophilia reflects allergen exposure, although this hypothesis has never been validated, and values quantifying the number of mucosal eosinophils that can be regarded as a normal finding are lacking. The aims of this study were to determine the number of intramucosal eosinophils normally present throughout the colon and evaluate the relationship between colonic eosinophilia and seasonal allergen exposure. Eosinophils in the crypt epithelium and lamina propria were evaluated in 198 mucosal biopsy specimens obtained from the ascending (n = 98) and descending (n = 100) colon of patients with normal colonoscopic examinations. The cases were stratified into 12 groups, reflecting the months during which the samples were obtained, and the mean number of mucosal eosinophils was determined for each group. Daily air pollen counts were recorded, and the mean determined for each month. Fifty-five percent of mucosal biopsy specimens from the ascending colon contained eosinophils in the crypt epithelium, compared with only 5% of biopsy specimens from the descending colon (P < .001). Lamina propria eosinophils were, on average, 3 times more numerous in the ascending compared with the descending colon (P < .001). Mucosal eosinophils were slightly more numerous in samples obtained in April and May, corresponding to periods of highest pollen counts, but this relationship was not significant (P > .05). We conclude that intramucosal eosinophils are commonly present in the proximal colon but show only mild fluctuations with ambient allergen exposure.