OBJECTIVE: To assess the activation grade of intestinal eosinophils in patients with eosinophilic gastroenteritis (EOG), ulcerative colitis (UC), Crohn's disease (CD), and controls by immunohistochemistry. METHODS: Cecal biopsies were collected from healthy controls and from patients with EOG, CD, UC, and other noninflammatory GI diseases. Immunohistochemistry was performed in sequential sections stained with monoclonal antibodies directed against eosinophil cationic protein (ECP) or eosinophil protein X (EPX) stored in eosinophil granules (EG1) and that secreted by activated eosinophils (EG2). The ratio of EG1 to EG2-positive eosinophils expressed as percentage of lamina propria cells was calculated. ECP and EPX were measured in serum and feces. RESULTS: The percentage of EG1 and EG2-positive lamina propria cells was elevated in EOG and slightly, but not significantly, in UC. The ratio of EG1 to EG2-positive cells was decreased in CD, UC, and other patients as compared to healthy controls. Particularly low EG1 to EG2 ratios were found in EOG. Correspondingly, fecal and serum levels of ECP and EPX, respectively, were highest in patients with EOG. The EG1 to EG2 ratio was negatively correlated with fecal ECP and EPX levels. At sites of actively inflamed mucosa, the EG1 to EG2 ratio was lower than in noninflamed tissue. CONCLUSIONS: Our data strongly suggest that the EG1 to EG2 ratio may be a marker of tissue eosinophil activation. Low ratios (<1) indicate eosinophil activation, whereas ratios > or =1 are found in healthy controls. Furthermore, we show that EOG is characterized by a pronounced intestinal eosinophil accumulation and activation, whereas in CD and UC, eosinophils seem to be activated but their number is not or only slightly elevated compared to controls.