During a controlled clinical trial of various diets in ulcerative colitis the levels of circulating eosinophils in the peripheral blood and infiltration of rectal biopsy specimens with eosinophils were studied. In some patients there was a rise in the circulating eosinophil count during a clinical relapse of the disease and the count was significantly higher when there was histologic evidence of inflammation than when the biopsy appearances were normal. The eosinophil count would not be used to predict the clinical course of the disease or to detect those patients who were likely to respond to a milk-free diet. The findings with tissue eosinophils were similar. Although some correlation was found between circulating and tissue eosinophils it was not close. The significance of the findings in relation to the possibility that allergy may be of importance in the etiology of ulcerative colitis is discussed.