Inflammatory bowel disorders are characterized by an accumulation of eosinophilic granulocytes, mast cells, lymphocytes and neutrophilic granulocytes in the intestinal mucosa. The aim of this study was to examine the concentration of eosinophilic granulocytes in the blood of patients during active ulcerative colitis in comparison with patients during remission and apparently healthy control subjects. Besides counting, the activity grade of eosinophilic granulocytes has been studied by estimation of their degranulation product eosinophil cationic protein. Subjects with active ulcerative colitis could be distinguished from patients with quiescent ulcerative colitis by establishment of the eosinophil cationic protein concentration, neutrophilic granulocyte count, erythrocyte sedimentation rate, C-reactive protein and albumin concentration. After two weeks of corticosteroid treatment, serum eosinophil cationic protein concentrations and eosinophil counts in blood were significantly decreased. A decrease in blood eosinophil count was accompanied by a decrease in eosinophil cationic protein concentrations in serum in most subjects with ulcerative colitis. After twelve weeks of corticosteroid administration, serum albumin concentrations were significantly increased, whereas serum concentrations of C-reactive protein were significantly decreased. During treatment with corticosteroids, serum eosinophil cationic protein concentrations and blood eosinophil counts are appropriate laboratory markers to detect the effect of medication in the course of ulcerative colitis.