Activation of eosinophil granulocytes (eosinophils) seems to contribute to the pathophysiology of several inflammatory conditions. This process was evaluated in 18 patients with ulcerative colitis and in 18 healthy controls using intraluminal segmental perfusion of the sigmoid colon and rectum and immunoanalysis for eosinophil cationic protein (ECP) in the perfusate. Immunohistochemistry for eosinophils and neutrophils was made in simultaneously taken biopsies and in biopsies from surgical specimens taken from additional 10 patients. The mucosal release of ECP was increased severalfold in patients with UC. The bowel biopsies demonstrated a lamina propria infiltrated with eosinophils. The degree of eosinophil activation/degranulation was related to the intensity of the inflammatory reaction. Activated eosinophils and extracellular deposits of ECP were, in particular, seen in crypt abscesses and in areas with damaged surface epithelium. Since ECP is highly cytotoxic, its release at the site of inflammatory bowel lesions might reflect a potential pathophysiological mechanism.